The Kiinde Kozii heats bottles faster than most other warmers, and it’s one of the few that doesn’t need fresh water every time. But like most bottle warmers, it can overheat milk and has mediocre reviews.
The thing that sets the Kozii apart from its competitors is that its circulating water bath heats bottles faster using lower temperatures than most other warmers. It’s the only warmer that lets you heat multiple bottles back to back without adding water or waiting for the unit to cool down. And the water drains from the warming chamber at the end of each cycle, so the bottle won’t continue to heat if you don’t retrieve it immediately. It can thaw frozen breast milk, but simpler methods do a better job. Like all the bottle warmers we found, the Kozii has unimpressive user ratings (3.5 stars out of five on Amazon), and though it is fast, it can overheat breast milk if you aren’t careful.
If you heat bottles only occasionally, or don’t mind extra effort for a slightly faster warm time, we recommend The First Years Simple Serve Bottle Warmer (formerly called the Quick Serve). It’s the fastest warmer we tested, heating a four-ounce bottle of refrigerated formula or breast milk in a little under 3 minutes (our pick takes 5½ minutes), and an 8-ounce bottle in 4½ minutes (our pick takes 7 minutes). As with the Kozii, you can heat multiple bottles in a row without a cooldown period. But you have to measure a precise amount of water for each cycle, which we found cumbersome—in comparison, our pick skips this step with a design that drains the warming chamber between cycles. The Simple Serve can’t defrost bags of frozen breast milk. And even when heating milk or formula to the right temperature, the bottle parts tend to come out quite hot and steam can build up in the nipple reservoir, so make sure it all feels cool to the touch before serving. Like the Kozii, and almost all other bottle warmers, the Simple Serve can overheat milk or formula if you use it improperly or leave it in too long.
Table of contents
Why you should trust us
To learn how to properly prepare and warm formula and breast milk, we spoke with Dr. Jenny Thomas, a pediatrician and breastfeeding medicine specialist and executive board member for the American Academy of Pediatrics’s (AAP) Section on Breastfeeding. We also read studies about the composition of breast milk and how it changes when heated. For additional information on handling formula, we corresponded with representatives from the Infant Nutrition Council of America, a trade group of manufacturers and marketers of infant formula, and consulted guidelines from the World Health Organization (PDF) and Centers for Disease Control.
We also read dozens of discussions about bottle warming on BabyCenter, What to Expect, and other pregnancy and baby websites and Facebook groups, and informally polled parents we know about if and how they heat baby bottles.
I’m the research editor for The Wirecutter, assisting our writers and editors with product research and reporting for more than 100 guides to date. I’m also the mom of three kids under the age of four, and I’ve pumped and stored breast milk to use for bottle feedings.
Who should get this
No bottle warmer will take all the guesswork or effort out of heating a bottle, and you’ll always need to check the final temperature.
After 10 hours of research it was clear to us that most people don’t use or need a dedicated bottle warmer. They aren’t especially useful for most people, and, at best, are only moderately more efficient than warming bottles under the tap or in a bowl of hot water. If you are considering this purchase, we have two ideas for you to try before you buy anything.
First, try it cold. Whether your baby drinks bottles of formula or breast milk, there’s no nutritional or medical reason to warm it to a specific temperature before feeding. “It can be served right out of the refrigerator. If the kids will take it cold, that’s fine,” Dr. Jenny Thomas told us. If you have a preterm baby or one with other health concerns, talk to your pediatrician about optimal bottle preparation. But if your baby is happy drinking unheated milk, problem solved.
Second, try heating with warm water. There is some technique to this, as we outline in the section below about how to heat breast milk or formula (with or without a dedicated bottle warmer). But the bottom line is the bottle warmers we tested are only moderately faster and easier than methods suggested by the AAP, Infant Nutrition Council of America, and many other infant care resources—essentially, warming bottles in a bowl of hot water or by running them under your tap. (Not in the microwave or on the stove.)
Bottle warmers aren’t especially useful for most people, and, at best, only moderately more efficient than warming bottles under the tap or in a bowl of water.
If you’ve tried those methods already and are still dissatisfied—understandable if you need to warm many bottles throughout the day, in a room without access to a hot water tap or stove, or for more than one baby at a time—then you might find a bottle warmer useful. Some caregivers even consider them indispensable. But set your expectations realistically: No bottle warmer will take all the guesswork or effort out of heating a bottle, and you’ll always need to carefully check the final temperature before serving it to the baby.
How we picked
Based on what we learned from our research and expert interviews, we determined a good bottle warmer should:
- Heat quickly (without overheating). A good bottle warmer should reliably reach the recommended serving temperature of 98 ˚F, heating the milk evenly and avoiding very hot spots or steam buildup. The warmer ideally should not heat any of the milk much above 104 ˚F, to avoid burn risk as well as the risk of altering the composition of breast milk. (Spoiler: We were disappointed in tests to find almost all of the warmers can overheat breast milk.) In researching how to safely heat breast milk and formula, we learned bottle warming should take less than 15 minutes, to guard against possible bacteria growth.
- Be more convenient than simple hot water. The warmer should heat the bottle more quickly, or require less effort—or, ideally, both!—than running the bottle under the tap or heating it in a bowl of water. The warmer shouldn’t require a complex set of steps or constant babysitting to use. (Another spoiler: The bottle warmers weren’t much faster or easier.)
- Be as foolproof as possible. The warmer should have clear instructions, easy-to-set timers and controls, and a clear signal when the warming cycle has ended.
- Fit a range of bottle sizes, shapes, and materials. You may use narrow or wide bottles, big or little bottles, silicone, plastic, or glass bottles (and you might use different types at different stages of your baby’s development). The warmer should accommodate as many of these shapes, sizes, and materials as possible.
- Allow back-to-back cycles. Some people use warmers because they need to heat bottles for more than one baby. A good warmer should let you run a second cycle with little or no wait time. Ideally the warmer should also maintain consistent heating temperatures for consecutive cycles.
- Be easy to maintain. The warmer should be easy to clean and store.
- Be inexpensive. As a single-purpose machine that you’ll most likely use for only a year or so, a bottle warmer shouldn’t cost more than $50, and ideally much less.
- Get good reviews. This requirement proved difficult. All bottle warmers have relatively poor Amazon reviews, with only 3.5-star ratings (out of five), on average. After spending 10 hours on research and another 10 hours testing and using bottle warmers, and comparing them with simply using hot water, we can see why.
After determining our criteria, we looked at bottle warmer reviews on sites like BabyGearLab and The Night Light, and bottle warmers on sites like Amazon, BuyBuy Baby and other retailers, coming up with a list of 18 of the most popular models.
All the warmers we found use water as the heat source—either a water bath in which you submerge the bottle, or by creating steam. We eliminated warmers with unnecessary extra features like attached insulated boxes (for keeping bottles cool) and Bluetooth connectivity. We also eliminated warmers that would take up a lot of space, or had few user reviews or suspicious reviews.
We then narrowed the list by focusing on warmers that performed better than others in comparative reviews or had the largest numbers of overall user reviews and positive reviews. This led us to six warmers we decided test:
How we tested
We tested six bottle warmers with seven popular baby bottles of different shapes, sizes, and materials1:
We prepared pitchers of powdered infant formula and refrigerated it until the liquid was around 40 °F.
We first ran several cycles with each warmer, using different bottles and quantities of formula, to see if there were any annoying flaws that would immediately disqualify the machine. We eliminated one warmer right away because it leaked water all over the counter, and another because it took over 10 minutes to warm a small bottle—and then wouldn’t turn back on.
We then warmed more bottles in the remaining four warmers. We filled small bottles with 4 ounces of formula, and larger bottles with 8 ounces of formula. The formula was between 40 °F and 50 °F at the start of each cycle, and we placed the probe near the center of the bottle, about halfway deep into the liquid. Using a ThermoWorks Dot thermometer inserted through the nipple of each bottle, we tracked the temperature of the milk as it heated, recording the temperature every 30 seconds as it rose. We also periodically measured the temperature at the surface of the milk and near its sides and bottom, and in the nipple reservoir, to see if any areas of the milk heated more quickly. (Because warmer liquid rises above cooler liquid, the surface temperature of the milk tended to be 10 to 15 degrees warmer than the center.)
After each cycle completed, we removed the bottle and noted the temperature at the surface of the milk (and of the steam in the nipple reservoir, if steam was produced). We then shook the bottle gently five times and took the temperature at the center of the milk. We gently shook it for about 30 seconds more, and then noted the final temperature. (All bottle warmers instruct you to shake the bottle for 30 seconds after the cycle ends.)
We noted how close each cycle heated the bottles to the desired temperature (around 98 °F). We considered warmers that hit 88 to 104 °F to be successful, because it’s better to underheat than overheat the bottle. (We knew that 104 °F is the temperature at which breast milk could begin to be altered; at that temperature it feels warm—not burning hot—to the touch. For comparison, a drinkably hot cup of coffee is typically around 130 °F). We recorded if any portions of the milk reached temperatures above 104 °F or significantly higher, and noted how easy it was to accidentally overheat using each model. We also noted whether the outside of the bottles felt hot once they came out of the warmer.
We logged how long each cycle took to heat the bottle to 98 °F, and compared those times with heating bottles under the tap or in bowls of hot water, using the methods outlined in how to safely heat breast milk or formula.
Minutes to warm
Minutes to warm
|Tap water||120 °F||5:00||8:30|
|Kiinde Kozii||140 °F||5:30||7:00|
|Bowl of hot water||150 °F (starting)||6:30||7:30|
|Chicco Bottle Warmer||180 °F||6:00||9:00|
|First Years Quick Serve||212 °F||3:00||4:30|
|Chicco NaturalFit Digital||212 °F||5:30||6:30|
We paid attention to how difficult it was to figure out the cycle settings. Some warmers only require you to turn the machine on, or select a bottle size, shape, material, and starting temperature, and then use thermostats to determine when to shut it off. Other machines require you to choose a time setting based on your own analysis of those factors, making it easier to miscalculate. Some machines require measuring a specific quantity of water rather than just filling up the reservoir, which also added complexity.
Based on these tests, we were able to narrow the field to two top warmers, which we then tested by heating bottles of breast milk (we had used formula up to this point). We first defrosted frozen bags of breast milk in the warmers, noting if they melted evenly and if portions of the milk overheated. We then heated 4-ounce bottles of refrigerated breast milk in each machine to determine if there were any differences in how breast milk heated versus formula. We found that breast milk bottles heated at similar speeds to formula bottles.
Our pick: Kiinde Kozii Breastmilk Warmer & Bottle Warmer
The Kiinde Kozii heats bottles faster than most other warmers, and it’s one of the few that doesn’t need fresh water every time. But like most bottle warmers, it can overheat milk and has mediocre reviews.
Though it shares many of the flaws we found across all bottle warmers, the Kiinde Kozii Breastmilk Warmer & Bottle Warmer heats a range of bottle sizes reliably and fairly quickly, and has a few features that make it more convenient to use than its competitors.
The Kozii is among the fastest warmers we tested, heating our 4-ounce bottles in about 5½ minutes and 8-ounce bottles in 7 minutes. The only faster performers were steam warmers (like our budget pick), which use much hotter temperatures. The Kozii heated bottles of formula and breast milk faster than any other water-bath warmer we tested because it uses a flowing 140 °F water bath instead of a still water bath. This causes “forced convection” (the same principle behind stirring soup or sous vide circulators), and it transfers heat more efficiently than still water. The stagnant water-bath warmers, like the Chicco Bottle Warmer, typically use hotter temperatures (the Chicco gets up to 180 °F), but heat more slowly. Compared with a simple bowl of hot water, the Kozii was faster by a minute on the 4-ounce bottle and 30 seconds on an 8-ounce bottle.
The Kozii is one of the easiest warmers to use because it doesn’t require measuring a precise amount of water or fiddling with multiple settings. You add water to the basin so it rises above the pump valve and turn the dial to the appropriate time setting for the bottle you want to warm—that’s simpler than setting a digital timer on competing models. An advantage to the Kozii is that the straightforward dial timer allows you to easily add another minute or two if you need to heat the bottle a little longer. With other warmers, like the First Years Simple Serve and Chicco NaturalFit Digital, you need to start a full new cycle.
The Kozii’s design allows you to repeat heating cycles again and again, as long as the water level is high enough to maintain circulation. This is because of a unique feature among warmers we tested: The Kozii drains the water from the warming basin into a reservoir once the cycle ends. That means the heat source (the hot water) is removed, so the bottle won’t continue to heat if you leave it in there like it does with some other warmers like the Chicco basic warmer. The Kozii’s drain/refill cycle is also much easier to use than other warmers (like the First Years Simple Serve and the Chicco NaturalFit Digital) that make you measure a precise amount of water for each use.
The Kozii fit all the bottle sizes and shapes we tested. We found all water-bath warmers, including the Kozii, tend to underheat the wide silicone Comotomo bottles and the tall, narrow Dr. Brown’s bottles.
The Kozii’s instructions include suggested warm times for bottles of different sizes, shapes, and materials. We found the recommended times were generally accurate and produced bottles with a final temperature of 88 to 104 °F, an appropriate range. We found an extra 30 seconds of heat can make a big difference, pushing the final temperature up 5 degrees or more. Letting a refrigerated bottle warm for a few minutes on the counter can also mean you’ll need to shorten the heat time, and running multiple cycles in a row (which means the water will be preheated) will also shorten the warm time. You’ll need to experiment with your bottles and the quantities of milk you heat to find the proper time setting for your needs. (If you’re concerned about the exact temperature of the bottle, the only accurate way to know it is to use an instant-read thermometer.)
Flaws but not dealbreakers
Despite its claims, the Kiinde Kozii can overheat formula and breast milk. To be fair, this is true of all the warmers we tested (except for the very low temperature, and too slow Avent warmer). Because the Kozii uses relatively cool 140 °F water—the steam warmers heat to 212 °F—the bottle will never get hotter than 140 °F. That’s way too hot to serve to a baby, of course, and it’s a temperature that may change or damage properties of breast milk, but it’s not as hot and potentially hazardous as those produced by some of the other warmers we tested.
Though the Kozii’s kitchen-timer-style dial is easy to use (you simply turn it to the time setting you want, such as between 5 and 6 for 5½ minutes), because it’s so small, it’s easy to accidentally set it slightly shorter or longer than you mean to.
The Kozii claims it can defrost bags of frozen breast milk, but we don’t recommend it in the Kozii or any other warmer. When we tested this, it took about 5 minutes to fully defrost and warm a 4-ounce bag. But we found the temperature of the milk was very uneven during melting and warming; some portions of the milk reached over 120 °F while the rest remained frozen. If you’re concerned about not exposing breast milk to high temperatures, defrost it in the refrigerator or in a bowl of hot water.
Some people may find the loud ticking of the Kozii’s timer, which sounds like a typical kitchen timer, annoying. And, frustratingly, the Kozii doesn’t have a ding, beep, or other alarm to let you know the cycle has ended—the end of the ticking is your signal. The company told us it hasn’t included a beep or other signal because most people find the ticking itself to be sufficient, and some people use the warmer near a sleeping baby. We would prefer an optional alert.
You might be surprised how much cleaning upkeep the Kozii requires. Kiinde’s instruction manual (PDF) says that you should clean the Kozii every two to three days by emptying the water chamber and refilling it with about a cup of fresh water mixed with some mild soap. Swirl it around, empty, and repeat. Every two months, descale the Kozii (to remove mineral deposits) by mixing a half cup of white vinegar with one cup of water and pouring the mixture into the machine. Let sit for 10 to 20 minutes, empty it, and rinse it.
Last, the Kozii is also one of the most expensive warmers we tested, but it comes with a one-year warranty.
Budget pick: The First Years Simple Serve Bottle Warmer (formerly Quick Serve)
The First Years Simple Serve Bottle Warmer (formerly Quick Serve) is a cheap, fast, and reliable warmer that requires a bit more work and has fewer features than our main pick. This steam warmer heated bottles more quickly, on average, than any other warmer we tried, warming four ounce bottles in a little under 3 minutes and eight ounce bottles in about 4½ minutes (our pick did this in about 5½ and 7 minutes, respectively). Like with the Kozii, you can heat multiple bottles in a row without a cooldown period. The First Years was the cheapest warmer we tested, but worked better than steam warmers that cost two or three times as much.
The First Years was the cheapest warmer we tested, but worked better than steam warmers that cost two or three times as much.
As with most steam warmers, you’ll have to measure a precise amount of water for each cycle, which is less convenient than the automatic refill/drain design of our pick. The First Years warmer uses a small, fiddly vial that can be difficult to fill accurately, which may be a detraction if you plan to use the warmer frequently. It’s otherwise simple to use, with a single “on” button that starts the heating plate, causing the water to boil and release steam to heat the bottle.
The length of the cycle is determined by the amount of water you add, and the First Years comes with instructions that include suggested amounts for various bottles and starting temperatures. As with all steam warmers—and unlike our pick—the bottles tend to feel quite hot immediately after warming and steam can build up in the nipple reservoir, so you’ll need to ensure they cool down properly before serving.
Unlike the Kozii, the First Years warmer can’t defrost frozen bags of breast milk. It requires regular cleaning of the steam chamber and heating plate with soap and water, and the instruction manual (PDF) has a process to clean white mineral deposits as well. The removable plastic parts can be washed in the dishwasher; the unit can’t be immersed. The First Years has a 90-day warranty as opposed to a full year for the Kiinde Kozii.
The Dr. Brown’s Deluxe Bottle Warmer is a steam warmer that has been lauded by other reviewers like those at The Night Light and New York Magazine. Unlike the other steam warmers we considered and tested, which require adding water each time, the Dr. Brown’s has an attached water reservoir that lets you load up enough water for several runs. The warming cycles are controlled by a digital timer (you set the length based on the size and temperature of your bottle) that is easy enough to use. But we found the water reservoir leaked all over the counter if we tipped or moved the warmer even slightly. It also won’t fit very wide bottles like the Comotomo.
Philips Avent claims its Nutrient-Preserving Bottle Warmer is safer for heating breast milk because it gradually warms bottles with flowing warm water and uses temperature sensors to prevent overheating. But the warmer actually uses a stagnant water bath, similar to the Chicco warmers we tested. (Philips Avent explained that its claim that the water “flows” is because it is heated from the bottom, which makes warmer water rise to the top; this is basic physics, and it happens with all water-bath warmers.) The Avent warmer does use cooler water (around 100 °F at the hottest setting) to heat bottles; this is good, in theory, because there’s no chance of overheating the milk. But such a low temperature makes for a very slow warm time; after 10 minutes, a refrigerated 4-ounce bottle had barely reached 80 °F. You have to add about a half cup of water to the basin, which you can theoretically use for multiple bottles, but if you want to run back-to-back cycles you’ll have to replace it with fresh, cool water.
The Chicco Bottle Warmer is a straightforward water-bath warmer that looks like a tiny Crock-Pot. It has a simple switch—you can choose bottles or baby food (we didn’t test baby food). You dump in a half cup of water, put in the bottle, and turn the machine on. The warmer heats the water to about 180 °F, which is 40 degrees hotter than the Kozii, but it uses a thermostat to gauge when the bottle is hot enough, which takes out some guesswork. It took about 6 minutes to heat a 4-ounce Avent bottle and 9 minutes to heat an 8-ounce Avent bottle to 90 °F before going to “keep warm” mode. But it also significantly underheated several bottles, switching to “keep warm” when the silicone Comotomo bottle was at only 60 °F. You have to wait about 15 minutes before running another heat cycle, which makes it hard to warm multiple bottles. This warmer wasn’t any faster than the Kozii. It was one of the simplest products we tried and it’s unfortunate the heating performance was such a weakness compared with our pick’s.
The Chicco NaturalFit Digital Bottle & Baby Food Warmer offers more settings than the basic Chicco warmer (you select both the bottle size and temperature), and you have to add water (2 or 3 tablespoons) for each use. It rapidly boils the water, heating the bottle mostly with steam. It worked consistently, heating 4-ounce bottles in about 5 minutes, and 8-ounce bottles in 6 minutes. But it doesn’t offer any advantage over the First Years warmer, which is a bit faster and much cheaper.
We didn’t test the Boon Orb bottle warmer, because BabyGearLab found it quickly overheated bottles, and you can’t use it with glass bottles. We didn’t test the Cuisinart Baby Bottle Warmer/Night Light either, because it performed poorly in BabyGearLab’s tests and is more expensive than the First Years warmer. We also didn’t test the Munchkin Timer Saver or Munchkin Speed or Nuby 2-in-1 warmers because they are more expensive than the First Years but don’t offer any extra functionality.
We didn’t test the First Years Night Cravings and Munchkin Night and Day bottle warmers because they have attached insulated boxes for storing a bottle (with your own cooler pack); we think this is unnecessary. We also skipped the Baby Brezza Safe & Smart warmer, which has Bluetooth and can be controlled by an app, because it has few reliable reviews and is too expensive for a single-use appliance. We also didn’t test the Tommee Tippee warmer because it lacks an automatic shutoff.
How to heat breast milk and formula (with or without a bottle warmer)
If you’re heating formula or breast milk for a baby, the final serving temperature should be around 98 °F or cooler. This is ideal because it’s around body temperature (the temperature of breast milk when nursing directly), and is also cool enough to be free from any risk of burning a baby’s mouth or body.
Safely heating breast milk
Breast milk contains bioactive components like white blood cells, immunological proteins, beneficial microbes (probiotics), and prebiotics (food for probiotics), to name just a few, that are sensitive to changes in temperature, storage, and other factors.
“Human milk is really meant to go from breast to baby. Any time that we put it into a container, or expose it to anything but the baby’s mouth, there are always changes that happen,” Dr. Thomas told us. In particular, heating breast milk above its native temperature (around 98.6 °F) can alter or destroy certain bioactive components. “There are some ingredients of human milk that are infection-fighting and are stable under lots of different conditions; you can do lots to them and you don’t destroy them. There are other ingredients, like proteins, that are denatured at too high a temperature.”
Exactly what temperature is too high? Because breast milk is so complex, this is hard to answer. A study (PDF) from the 1980s suggested certain bioactive properties begin to change at temperatures as low as 104 °F to 122 °F, with more damage occurring as the temperature increases past 140 °F. Though it’s important to protect breast milk from heat damage, Dr. Thomas cautioned that even overheated breast milk maintains a great deal of its nutritional and anti-infective benefits and is superior to formula. When donor milk is processed for use in hospitals, for example, it is pasteurized at 144.5 °F.
Dr. Thomas also warned you shouldn’t vigorously shake bottles of expressed breast milk, because rough motion can also damage components of the milk. Gently swirl it instead.
Safely heating formula
Formula doesn’t have the same heat sensitivities as breast milk, but The Infant Nutrition Council of America told us that warming for all types of formula and breast milk should take less than 15 minutes—that’s a measure only of the time that it’s heating—to reduce the opportunity for possible bacteria to grow. The standard guidance from INCA and the AAP is to consume the formula within 1 hour of preparation, or refrigerate it immediately and serve it within 24 hours.
The WHO (PDF) and CDC advise that powdered formula be mixed with water that is no cooler than 158 °F to kill bacteria that could be present in the powder. That means you’ll need to initially prepare the formula much hotter than you could safely serve a baby, and wait for it to cool down (either in the refrigerator, in cool water, or in an ice bath). The Infant Nutrition Council of America says you can prepare a larger quantity of powdered formula in advance, cover it, and store it in the refrigerator below 40 °F for up to 24 hours. You can then prepare and rewarm bottles as needed.
Liquid-concentrate formula (which you mix with water) and ready-to-feed formula (which comes in preprepared bottles) do not need to be prepared with hot water or specially heated. We plan to explore the pros and cons of all types of formula in a future guide.
Whether you’re mixing with powdered or liquid-concentrate formula, the water you use needs to be safe from contaminants. Speak with your baby’s pediatrician and local water department to find out whether your tap water needs to be boiled or purified before mixing.
Safely heating breast milk or formula without a dedicated bottle warmer
Bottles of both formula and breast milk can be easily heated without a bottle warmer in roughly the same amount of time it takes our pick to do the job.
The first thing we’d suggest to anyone new to warming bottles is to simply place the bottle in a bowl of hot water, submerging as much of the bottle as possible. This uses much less water and requires less effort than warming it under hot tap water. We heated about 3 cups of water to about 150 °F in a kettle, and then poured it in a bowl. Once immersed, it took about 6½ minutes for a 4-ounce refrigerated bottle of formula to warm to about 98 °F, and 7½ minutes for an 8-ounce bottle to heat to that temperature (that’s slower but still comparable with our bottle warmer picks’ heating times).
You should never heat bottles in the microwave. You also should not warm bottles in boiling water, or directly on the stove.
The other option is to hold the bottle under hot, running tap water for a few minutes, gently swirling or shaking the bottle to heat it evenly. Tap water in US homes typically tops out around 120 °F, and we found it took about 5 minutes to heat a refrigerated 4-ounce bottle of formula to around 98 °F, and about 8 minutes to heat an 8-ounce bottle. A downside is that it wastes a lot of water—two gallons or more, in our tests—and requires you to stand at the sink for 5 or more minutes (you could also just place it under a stream of hot water, though it will take more time to heat if you’re not also swirling the bottle).
No matter how you warm the bottle, gently shake it after warming and test the temperature of the liquid by placing a few drops on the inside of your wrist. The liquid should feel no warmer than body temperature (meaning, it shouldn’t feel particularly warm on your skin—just tepid). Carefully check that no part of the bottle or nipple is too warm for the baby to touch, and that no steam has built up in the nipple reservoir.
All experts and medical organizations we consulted say you should never heat bottles in the microwave, as it can lead to hot spots in the milk, cause steam to build up in the nipple, or even make the bottle explode. You also should not warm bottles in boiling water, or directly on the stove, which could quickly overheat the bottles.