Boil water at a full rolling boil, then let it cool for no longer than five minutes before mixing it with powdered infant formula. That five-minute window is critical: the water needs to still be very hot (around 158°F/70°C) when it hits the powder to kill bacteria that can live inside the formula itself. Once mixed, you’ll need to cool the bottle before feeding.
Why Boiling Matters for Powdered Formula
Powdered infant formula is not sterile. It can contain harmful bacteria, most notably Cronobacter, which causes rare but serious infections in newborns. The boiling step serves two purposes: it purifies the water, and the residual heat after cooling for five minutes is hot enough to kill germs in the powder. This is why you don’t just boil and then let the water cool completely before mixing. The water itself isn’t the main concern. The powder is.
This step is especially important for babies under 2 months old, babies born prematurely, and babies with weakened immune systems. For these infants, the CDC specifically recommends the boil-then-wait-five-minutes method every time you prepare a bottle.
Liquid (ready-to-feed) formula is manufactured to be sterile and doesn’t carry the same Cronobacter risk. If you want to skip the boiling process entirely, ready-to-feed formula is the simplest alternative, though it costs more.
Step by Step: Boiling and Mixing
Start with fresh cold tap water. If your home has older pipes that might contain lead, run the cold tap for 30 seconds to a minute before collecting water. This flushes standing water that may have picked up higher concentrations of lead or other metals from the plumbing.
Bring the water to a full, rolling boil. If you’re using an electric kettle, let it boil until it switches off automatically. A stovetop pot works just as well. Once the water reaches a rolling boil, turn off the heat and set a timer for five minutes. At the five-minute mark, the water will have cooled to roughly 158°F/70°C, which is the temperature needed to neutralize bacteria in the powder.
Add the correct amount of powder to the hot water following the measurements on the formula label. Swirl or shake gently to dissolve. The bottle will now be far too hot for your baby. Cool it quickly by holding it under cold running water or placing it in a bowl of cold water. Test the temperature by dropping a small amount on the inside of your wrist. It should feel lukewarm, not warm.
Which Water to Use
Regular tap water is fine for most families in areas with treated municipal water. If you’re unsure about your local water quality, your water utility’s annual report (sometimes called a Consumer Confidence Report) will list contaminant levels.
Distilled or purified water can be used without boiling it first, since the purification process has already removed bacteria and contaminants. However, if you’re preparing powdered formula, you still want the water hot enough to kill any germs in the powder itself. So even with distilled water, boiling and cooling for five minutes remains the safest approach for high-risk infants.
If you use bottled water, check the label. Sodium should be no higher than 200 milligrams per liter (listed as “Na” on some labels), and sulfate should be no higher than 250 milligrams per liter (sometimes listed as “SO” or “SO4”). Mineral waters and sparkling waters are not appropriate for infant formula.
Fluoride Concerns
Some parents worry about fluoride in tap water affecting their baby’s developing teeth. The risk of enamel fluorosis from fluoridated tap water mixed with formula is low. Advanced fluorosis is extremely rare even in communities that have fluoridated water for decades, and milder forms are cosmetically unnoticeable. The critical window for fluorosis in permanent teeth occurs later in childhood, when most kids have moved off formula. That said, if this concerns you, using water labeled as purified, distilled, deionized, or reverse osmosis filtered will give you a low-fluoride option.
Storing Boiled Water
If you want to have boiled water ready to go, you can prepare it once a day. After boiling, pour the water into a clean, covered container, label it with the date and time, and store it in the refrigerator. Discard any unused boiled water after 24 hours and start fresh. When you’re ready to make a bottle, you’ll need to reheat the stored water to a full boil again (or at least back above 158°F) if you’re using it with powdered formula for a high-risk infant, since the purpose of the heat is to treat the powder, not just the water.
Storing Prepared Formula
Once you’ve mixed the formula, use it within two hours at room temperature. If your baby starts a bottle but doesn’t finish it, discard any remaining formula within one hour, because bacteria from your baby’s mouth enter the liquid during feeding. Prepared formula that hasn’t been offered to the baby can be stored in the back of the refrigerator for up to 24 hours.
Common Mistakes to Avoid
- Letting boiled water cool too long before mixing. If you wait 30 minutes instead of five, the water may drop below the temperature needed to kill bacteria in the powder. The five-minute window exists for a reason.
- Boiling water repeatedly. Use fresh water each time. Re-boiling concentrates any minerals or contaminants already in the water.
- Adding powder before water. Always pour the water first, then add powder. This ensures accurate volume and proper mixing.
- Skipping the wrist test. Formula that feels comfortable to your hand can still scald a baby’s mouth. The inside of your wrist is more sensitive and gives a better read.
- Microwaving bottles to reheat. Microwaves heat unevenly and can create hot spots in the liquid that burn your baby’s mouth even when the outside of the bottle feels fine. Use warm running water or a bottle warmer instead.