Preparing formula safely is one of those topics where well-meaning but incorrect advice is genuinely widespread. From social media groups to family members who raised children a generation ago, you will encounter countless shortcuts and workarounds. Many of them carry real risk. This guide explains the evidence-based, NHS-correct method and addresses the most common points of confusion.
Powdered infant formula is not sterile. It can contain bacteria — most notably Cronobacter sakazakii and Salmonella — which can cause serious illness in young infants. These bacteria are killed by heat. This is why the NHS method specifies making formula with water that has been boiled and cooled to no less than 70°C: at this temperature, pathogens in the powder are destroyed.
The risk is highest in the first two months of life, but the same method should be used throughout the first year unless your baby is drinking ready-made liquid formula, which is sterile.
Boil fresh tap water and allow it to cool for no more than 30 minutes. This brings it to approximately 70°C. Do not use water that has been left to cool for longer — it will have dropped below the temperature needed to kill bacteria. Do not use water that has already been boiled once and re-boiled; re-boiling increases the concentration of minerals.
Wash your hands thoroughly with soap and water before preparing any feed.
Clean and sterilise all equipment — bottles, teats, caps, and any measuring equipment — before use.
Pour the correct amount of water into the sterilised bottle first. Always add water before powder. Adding water to powder first can alter the concentration.
Add the exact number of level scoops of formula powder as instructed on the packaging. Use the scoop provided with that tin — scoops vary between brands. Level off each scoop with a clean, dry knife. Do not pack the powder down or heap the scoop.
Seal the bottle and shake until the powder is fully dissolved.
Cool the feed quickly. Hold the bottom of the bottle under cold running water or place in a bowl of cold water. Do not leave it sitting on the counter to cool — this gives bacteria time to multiply if any survived the 70°C step.
Test the temperature by shaking a few drops on the inside of your wrist. It should feel warm, not hot.
Use the feed immediately, or store correctly (see below).
Using cooled boiled water that has been sitting for more than 30 minutes. By this point the water is likely below 70°C and will not kill bacteria in the powder. If you are using a flask to keep water at the correct temperature, it must maintain 70°C or above.
Making up formula with bottled water. Bottled water is not recommended for making up infant formula as it is not sterile and may have high mineral levels. If tap water is unavailable, bottled water can be used but should still be boiled first.
Using a microwave. Microwaves heat liquid unevenly, creating hot spots that can burn your baby's mouth. They do not heat the formula to a consistent temperature throughout. Never heat formula in a microwave.
Not following the exact ratio. Under-concentrated formula (too little powder) means your baby does not get adequate nutrition. Over-concentrated formula (too much powder) puts strain on their developing kidneys. Always follow the manufacturer's instructions precisely.
Using the same scoop between different formula brands. Scoop sizes differ between manufacturers. Always use the scoop that came with the tin you are currently using.
The NHS recommendation is to make each feed fresh when it is needed. However, if this is not practical, made-up formula can be stored in the back of the fridge (not the door) at 5°C or below for up to 24 hours.
To cool a freshly made feed for storage: cool it quickly under cold running water or in a bowl of cold water within 30 minutes, then refrigerate immediately. When you need to use it, rewarm it by placing the bottle in a bowl of warm water or using a bottle warmer — never in a microwave — and use within two hours of removal from the fridge.
Do not store made-up formula at room temperature. Do not reheat a feed more than once. Discard any formula left in the bottle after a feed, since bacteria from your baby's mouth will have entered the bottle.
Ready-made formula comes in cartons or small bottles and is sterile until opened. It is the safest and most convenient option when you are away from home, travelling, or in the middle of the night when making up a feed feels impossible. It is more expensive per feed than powder, so many families use it selectively.
Once opened, ready-made formula should be covered and refrigerated, used within 24 hours, and warmed using the same method as above.
When going out, there are two safe options: carry a flask of water kept at 70°C or above (vacuum flasks can maintain this temperature for several hours) and make up feeds as needed, or carry a carton of ready-made formula. Carrying a pre-made bottle of formula kept warm is not safe — formula held between 25°C and 40°C is an ideal environment for bacterial growth.
If you must pre-make a feed for an outing, make it fresh, cool it quickly, refrigerate it until you leave, and transport it in a cool bag with ice packs to keep it below 5°C. Use within four hours.
Filtered tap water (through a standard kitchen filter jug) should still be boiled before use. Filters remove some impurities but do not sterilise water or remove all harmful bacteria.
Getting formula preparation right from the start takes a few days of practice and then becomes entirely routine. The extra steps exist for good reason — and are well worth the effort.
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