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By the Peanutbean parent · checked against CDC, NHS and AAP guidance · how we fact-check

How to make up baby formula safely, step by step

Last verified: July 19, 2026

The short answer: boil fresh tap water and let it cool for no more than 30 minutes, so it stays at least 70C, which kills any bacteria that can be present even in sealed formula tins. Pour the hot water into the bottle first, then add the exact amount of powder the manufacturer specifies, no more and no less. Cool the bottle under cold running water, test it on your wrist, and throw away anything your baby does not finish within an hour of starting the feed.

Why the water temperature matters

Even sealed tins and packets of powdered infant formula can sometimes contain bacteria, and bacteria multiply fast at room temperature, more slowly but still in the fridge. Using water that is at least 70C when it meets the powder is what kills those bacteria (NHS: How to make up baby formula).

To get there reliably: fill the kettle with at least 1 litre of fresh tap water, never water that has already been boiled once, boil it, then let it cool for no more than 30 minutes. That window keeps the water at or above 70C (NHS).

The AAP frames the same safety concern slightly differently by age: for infants younger than 2 months, babies born prematurely, or babies with a weakened immune system, boiled water should be used to kill any microbes, then cooled for about 5 minutes before adding the formula (AAP: How to Safely Prepare Baby Formula With Water). For everyday feeds beyond that early, higher-risk window, both sources agree the core habit, clean water, boiled and properly cooled, is what protects your baby.

If you use a baby formula preparation machine, check that the water it dispenses is still at least 70C when mixed with the powder. A digital food thermometer can confirm this. If it reads below 70C, stop using the machine for now and go back to the kettle method, and consider contacting the manufacturer or Trading Standards about a possible fault (NHS).

Step by step: making up a feed

1. Clean and disinfect your work surface, then wash your hands (NHS).
2. Boil fresh tap water, then let it cool no more than 30 minutes.
3. Water goes in the bottle first, while still hot. Pour in the exact amount the manufacturer’s instructions specify, checking the level carefully (NHS; AAP).
4. Add the powder after the water, loosely filling the scoop that came with that specific formula, then leveling it with a clean, dry knife or the leveller provided. Do not mix and match scoops between brands (NHS).
5. Screw on the teat and cap, then shake until the powder is fully dissolved (NHS).
6. Cool the bottle under cold running water, with the lid still on, until it is safe to drink (NHS).
7. Test the temperature on the inside of your wrist. It should feel warm or cool, never hot (NHS; AAP).

The AAP adds one more option for the everyday routine: once your baby is past the higher-risk early weeks and boiled water is not medically necessary, a bottle can be made with room-temperature tap water and powder just before a feed, with no extra cooling or warming needed (AAP). Follow whichever approach your own formula’s instructions and your pediatrician recommend for your baby’s age and health.

Never add extra water, and never add extra powder

This is the detail worth repeating on its own, because it goes against an understandable instinct to stretch a feed further. Adding extra water to save on formula or to add “just a little water” alongside a feed can disturb your baby’s electrolyte and mineral balance, including calcium, sodium and potassium, and can lead to a dangerous condition called water intoxication, in serious cases causing seizures (AAP). The NHS gives the same instruction from the other direction: never add extra water when making up a feed, since too little powder will not give your baby enough nourishment (NHS).

Adding extra powder is just as risky in the other direction. More powder than instructed can make your baby constipated or dehydrated (NHS). The manufacturer’s ratio of water to powder is not a rough guide, it is the safety line.

If the real issue is affording formula, both sources point the same direction: talk to your pediatrician, health visitor, local WIC office, food pantry, or another social service agency rather than diluting feeds yourself (AAP).

How long a made-up bottle keeps

• Once your baby starts a feed, finish or discard it within about an hour. The AAP is specific: prepared formula must be discarded within 1 hour of feeding it to your baby (AAP). The NHS puts it plainly too: if there is any made-up formula left in the bottle after a feed, throw it away (NHS).
• A made-up bottle that has not been given to your baby yet can go in the fridge for up to 24 hours (AAP).
• Never warm formula in a microwave. It heats unevenly and can burn your baby’s mouth even when the bottle feels fine on the outside (NHS).
• To cut down on how much gets thrown away, both organisations suggest making up feeds one at a time as your baby needs them, rather than batching several ahead (NHS).

What kind of water to use

Ordinary tap water is the recommended default in both countries. The NHS says to use freshly boiled drinking water from the tap, not water that has been boiled before, and not artificially softened water (NHS). The AAP adds that it is best to primarily use safe tap water that is fluoridated, with only occasional use of non-fluoridated bottled water (AAP).

Bottled water is not usually recommended for everyday formula feeds, since it is not sterile and can contain too much salt or sulphate. The exception is when your normal drinking water is unavailable or contaminated, in which case bottled water can be used, alongside any specific guidance from your local water safety authority (NHS).

When to check with your doctor

Formula preparation mistakes are usually fixable once you spot them, but get in touch with your pediatrician or health visitor if:

• your baby has ongoing diarrhoea, vomiting, or seems unwell after feeds, which can be a sign of contamination or an intolerance worth checking
• you are unsure how to afford or access formula and are considering diluting feeds to stretch them
• your baby was born prematurely, has a weakened immune system, or is younger than 2 months, since the extra boiling-water precautions matter most for this group

And as always: your own concern is reason enough to ask.

Keep reading

• Oral thrush in babies
• How much milk does my baby need?
• Sterilising bottles and feeding equipment
• Baby vitamin D: who needs it and how much
• All the first-year windows

Sources

• NHS: How to make up baby formula (page last reviewed 31 August 2023, next review due 31 August 2026)
• AAP: How to Safely Prepare Baby Formula With Water (last updated 25 February 2026)

Peanutbean provides general information for educational purposes only. It is not medical advice. Every baby develops at their own pace. Always talk to your pediatrician about your child’s health.