Formula should only be warmed once. If your baby hasn't yet drunk from the bottle and it has been kept properly in the fridge, you can take it out and warm it a single time. Once your baby has started feeding, or once a bottle has been warmed, any leftover formula should be thrown away rather than cooled and reheated for a later feed. Reheating the same formula more than once gives bacteria the chance to multiply, and that's the risk we're trying to avoid.

If you're standing at the kitchen bench at 2am wondering whether that half-finished bottle is okay to warm up again, this guide walks you through exactly what's safe, what isn't, and the gentlest way to warm a feed — all based on Australian infant feeding guidance.

Can you reheat baby formula?

Yes — but only under the right conditions, and only once. There are really two different situations parents mean when they ask this question, and they have very different answers.

Formula your baby hasn't touched

If you made a bottle, cooled it and stored it in the back of the fridge (not the door) at 5°C or below, and your baby has not fed from it, you can warm it one time when you're ready to use it. Prepared formula keeps in the fridge for up to 24 hours. Take out only what you need, warm it once, and use it straight away.

Formula your baby has already fed from

Once your baby's mouth has touched the teat, bacteria from their saliva enter the milk. Even if the bottle goes straight back in the fridge, that feed should be used within an hour and never reheated for later. When in doubt, throw it out — it's not worth the tummy upset.

Why you shouldn't reheat formula more than once

Warm milk is the ideal environment for bacteria to grow. Every time formula is warmed and then cooled again, it spends more time in the “danger zone” between about 5°C and 60°C where bacteria multiply fastest. Reheating a second or third time compounds that risk, which is why the advice is always to warm once and discard the rest. The NHMRC Infant Feeding Guidelines recommend preparing formula fresh for each feed whenever you can, precisely because it removes this risk altogether.

The safest way to warm a bottle of formula

How you warm matters just as much as how many times. The goal is gentle, even heat to body temperature — around 37°C.

• Stand the bottle in warm water. Place it in a jug or bowl of warm (not boiling) water for a few minutes, swirling gently to spread the heat.
• Use a bottle warmer. A dedicated warmer heats to a consistent temperature without hot spots — far more reliable than guessing with a kettle.
• Test before you feed. Shake the bottle and drip a little onto the inside of your wrist. It should feel lukewarm, not hot.

Never use a microwave. Microwaves heat unevenly and create scalding hot spots that can burn your baby's mouth and throat, even when the bottle feels cool on the outside. For more on why, see our guide to bottle warmer vs microwave.

Warming formula when you're out and about

Reheating gets trickier away from home, where there's no fridge or clean kettle handy. The safest approach when travelling is to carry pre-measured formula powder and a separate flask of cooled boiled water, then mix a fresh bottle only when your baby is hungry — so there's no reheating involved at all. A formula feeding flask keeps water at the right temperature for hours, and a portable warmer means you can bring a bottle to feeding temperature in the car or at the park. Browse the full range in our travel bottle warmer collection. For more tips, read how long formula can stay warm in a flask.

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How long does prepared formula last?

• Freshly made, unrefrigerated: use within 2 hours; discard after that.
• Made in advance and refrigerated (baby hasn't fed): up to 24 hours in the coldest part of the fridge.
• Warmed: use within 1 hour; never re-refrigerate or reheat.
• Baby has fed from it: use within 1 hour, then discard.

Australia's Raising Children Network has a helpful overview of making and storing formula safely. For a step-by-step routine, see our guide to preparing formula safely, and if you're weighing up devices, our portable bottle warmer comparison breaks down the options.

Frequently asked questions

Can you reheat formula twice?

No. Formula should only ever be warmed once. If a warmed bottle isn't used, throw it out rather than cooling and reheating it, because each reheat increases the chance of bacterial growth.

Can you put warmed formula back in the fridge?

No. Once formula has been warmed it shouldn't go back in the fridge. Use it within an hour of warming, and discard whatever's left.

How long can formula sit out before it goes off?

Freshly prepared formula left at room temperature should be used within 2 hours. Once your baby has fed from the bottle, use it within 1 hour and then throw the rest away.

Is it safe to reheat formula in the microwave?

No. Microwaves heat unevenly and create hot spots that can scald your baby's mouth. Warm bottles by standing them in warm water or using a bottle warmer instead.

Can I reuse formula my baby didn't finish?

No. Leftover formula from a bottle your baby has fed from should be discarded within an hour. Saliva introduces bacteria that multiply as the milk sits, even in the fridge.

What temperature should formula be for feeding?

Body temperature — around 37°C. It should feel lukewarm, not hot, when you test a few drops on the inside of your wrist. Cool or room-temperature formula is also perfectly safe to feed if your baby will take it.

This article is general information for Australian parents and isn't a substitute for advice from your child health nurse or GP. If your baby has specific feeding or health needs, check in with a health professional. Sources: NHMRC Infant Feeding Guidelines; Raising Children Network.