How to Make Up Formula Safely: The NHS Method

The NHS's method for making up baby formula comes down to one non-negotiable rule: mix the powder with water that's at least 70°C, straight from a kettle that's boiled less than 30 minutes ago. That temperature isn't about taste or texture — it's what kills the bacteria that can be present in powdered formula, even in an unopened, sealed tin. Everything else in this guide (the cooling, the timing, the throw-it-away-after-two-hours rule) exists to support that one principle.

Why the water temperature actually matters

Powdered infant formula isn't sterile. According to the NHS, even sealed tins and packets can sometimes contain bacteria, and these multiply fastest at room temperature. Water at 70°C or above kills that bacteria on contact, which is why the NHS is specific about it: don't use water that's cooled past that point, and don't use water that's been boiled and left to stand or reboiled from an already-once-boiled kettle.

How to make up a bottle, step by step

The NHS's method in practice:

1. Fill the kettle with at least 1 litre of fresh tap water — never water that's already been boiled once.
2. Boil it, then let it cool for no more than 30 minutes so it stays at 70°C or above.
3. Clean and wipe down the surface you're working on, and wash your hands.
4. If you're using a sterilised bottle, pour the hot water in first — checking the level against the marked measurements — before adding the powder. Adding water before powder, not after, is what gets the temperature right.
5. Add the formula powder using only the scoop that comes with that tin, levelled off with the built-in leveller or a clean, dry knife — don't pack it down or heap it.
6. Fit the teat and lid, then shake or swirl until the powder's fully dissolved.

Cooling it down safely

Because the water starts at 70°C+, the made-up bottle is too hot to feed straight away. Hold it (lid on) under cold running water, or stand it in a jug of cold water, until it's body temperature. Always test a few drops on the inside of your wrist before offering it — it should feel warm, not hot.

Storage and the two-hour rule

The safest approach, and the one the NHS recommends, is to make up each feed fresh, just before your baby needs it. If there's any formula left in the bottle after a feed, throw it away — don't save it for later or reheat it.

If you genuinely need to prepare a bottle in advance (for example, for a partner or carer to give later), cool it quickly and store it at the back of the fridge, then use it within 24 hours. Away from a fridge, a made-up bottle carried in a cool bag with an ice pack needs to be used within 4 hours; without an ice pack, that drops to 2 hours. A flask is fine for carrying hot water to make a fresh feed on the go, but it shouldn't be used to carry pre-made formula — if you can't make a feed fresh when you're out, a ready-to-feed liquid formula carton is the safer fallback.

Never adjust the ratio

It can be tempting to add an extra scoop for a hungrier baby, or a splash more water to make a tin last longer, but the NHS is explicit that you shouldn't do either. Follow the manufacturer's instructions on the tin exactly — brands vary in how much water and powder they specify. Extra powder makes the feed too concentrated, which can constipate or dehydrate your baby; extra water dilutes it, and too little powder means your baby isn't getting enough nourishment. If your baby seems hungrier than usual, the answer is a bigger or more frequent feed made to the correct ratio, not a stronger one.

Ready-to-feed formula as a backup

Ready-to-feed liquid formula — sold in small cartons, already sterile and at the correct concentration — doesn't need mixing or heating from cold, and it's a sensible thing to carry for days out, travel, or any situation where making up a fresh powder feed isn't practical. It's more expensive than powder for everyday use, but it removes the water-temperature and ratio steps entirely, which is exactly what you want when you're away from a kettle.

Sterilising your equipment

Bottles, teats and any other feeding equipment need to be sterilised — not just washed — every time, until your baby's at least 12 months old. Running items through a dishwasher cleans them but doesn't sterilise them. For the full breakdown of cold-water, steam and boiling methods and how long each keeps equipment sterile, see our steriliser buying and method guide.

Which formula should you actually use?

First infant formula (sometimes labelled "first milk") is what your baby needs from birth, and — unless a health visitor or GP advises otherwise — it's all they need for the whole first year. Follow-on formula, hungrier-baby formula and comfort formula are all marketed with claims about settling babies or keeping them fuller for longer, but none of that is backed by evidence, and follow-on formula specifically offers no benefit over first infant formula when introduced at 6 months. Specialist formulas — anti-reflux, hypoallergenic, lactose-free, soya — should only be used on the advice of a health professional, not chosen off a shelf. For the wider decision between breastfeeding and formula feeding, see our honest basics guide.

Common mistakes worth avoiding

A few habits are common but go against the guidance above: reboiling water that's already been boiled once, letting boiled water cool for longer than 30 minutes before adding it, packing formula powder tightly into the scoop instead of levelling it off, and microwaving a bottle to reheat or warm it (which can create dangerous hot spots even when the outside feels cool). None of these save meaningful time, and all of them work against the one thing the whole method is designed to do — kill bacteria and keep your baby's feed at a safe, even temperature.