There is a moment most new formula-feeding parents recognise: it is 3am, you are holding a crying baby with one hand, and you are trying to remember whether you have to wait for the kettle to cool or whether it needs to still be hot. Formula preparation instructions exist for very specific safety reasons — and in the sleep-deprived haze of new parenthood, they are also very easy to get muddled. This guide walks through safe formula preparation clearly and completely, so that when the 3am moment arrives, you already know exactly what to do.

Why Preparation Temperature Matters

This is the single most important thing to understand about formula preparation: powdered infant formula is not a sterile product. The manufacturing process cannot guarantee the complete absence of bacteria — in particular, Cronobacter sakazakii, a rare but serious pathogen. The way safe preparation eliminates this risk is by using water that is still above 70°C when it contacts the powder.

This is why guidance states to boil fresh water and use it within 30 minutes — not to let it cool for an hour or more, and not to use water that was boiled earlier and reheated. A freshly boiled kettle, used within 30 minutes, still meets the temperature requirement. Cold tap water, filtered water, or pre-boiled water that has fully cooled does not.

Step-by-Step: How to Make a Formula Feed

1. Wash your hands thoroughly with soap and water before handling any feeding equipment.
2. Boil fresh tap water — at least one litre, to ensure the kettle reaches a full boil. Do not use previously boiled water or water from a filter jug.
3. Allow to cool for no more than 30 minutes. The water should still be above 70°C when you use it.
4. Measure the correct amount of water into a sterilised bottle first — always water before powder.
5. Add the correct number of level scoops of formula powder using the scoop provided in the tin. Level each scoop with a clean, dry knife — do not pack, heap, or add extra scoops.
6. Close the bottle and mix by shaking or inverting gently until the powder is fully dissolved.
7. Cool the feed rapidly by holding the bottle under cold running water or placing it in a bowl of cold water. Test the temperature on the inside of your wrist — it should feel lukewarm, not hot.
8. Feed immediately. Discard any formula left in the bottle after a feed — never save a partially consumed feed.

Sterilising Feeding Equipment

All bottles, teats, bottle caps, and any equipment that contacts formula or the baby’s mouth should be sterilised before use for the first 12 months. Sterilising kills the bacteria that washing alone cannot fully remove from the small surfaces and joins of feeding equipment.

Sterilising methods

• Electric steam steriliser: Fast, reliable, and widely used. Follow the manufacturer’s instructions for water amount and cycle time.
• Microwave steam steriliser: Similar effectiveness to electric; ensure you follow specific instructions for your model.
• Cold water sterilising (chemical): Uses sterilising solution or tablets. Equipment must be fully submerged for the required time and rinsed before use.
• Boiling: Submerge equipment in a pan of boiling water for at least 10 minutes. Effective but can degrade some equipment faster.

After sterilising, bottles can be stored assembled (teat inverted inside the bottle) in the fridge for up to 24 hours if not used immediately.

Storing Prepared Formula

Whenever possible, prepare feeds fresh. If advance preparation is necessary — for example, for overnight feeds or when going out — these storage guidelines apply:

• Prepare the feed correctly using water above 70°C, then cool it rapidly under cold water
• Store immediately in the back of the fridge (not the door) at 5°C or below
• Use within 24 hours of preparation
• Do not freeze prepared formula
• When you need the feed, warm it by placing the bottle in a bowl of warm water or using a bottle warmer — never use a microwave, which creates hot spots that can burn the baby’s mouth
• Once warmed, use within one hour and discard any that remains

Ready-to-Feed Formula: When Convenience Justifies the Cost

Ready-to-feed (RTF) liquid formula is sterile, requires no preparation, and carries no risk of preparation error. It is considerably more expensive than powdered formula, but there are circumstances where it is genuinely worth the cost: for newborns in the first weeks when the stakes of preparation error feel highest, for travel, for illness in the caregiver, or for any situation where safe preparation is not practical. RTF formula poured into a sterilised bottle and used immediately is as safe as it is possible for formula to be.

Common Preparation Mistakes — and Why They Matter

These are the most frequently made formula preparation errors, each with a specific reason it matters:

• Using cooled or previously boiled water: Will not reach 70°C — does not eliminate formula powder bacteria
• Adding powder before water: Makes it harder to measure the correct water volume accurately
• Adding extra powder scoops: Makes formula too concentrated, straining baby’s kidneys and potentially causing dehydration
• Using less powder than instructed: Under-diluted formula does not provide adequate nutrition
• Saving leftover formula after a feed: Bacteria from the baby’s mouth enters the bottle during feeding — any remaining formula must be discarded
• Warming in the microwave: Creates uneven hot spots that can cause mouth burns even when the bottle feels cool to the touch

“”The preparation guidelines for powdered infant formula are not overcautious — they are precisely calibrated to address a real, if rare, microbiological risk. Following them carefully is one of the simplest and most effective things a parent can do for their baby’s safety.””

— Professor Alan Weaver, consultant paediatric dietitian and infant feeding researcher. Professor Weaver has contributed to national infant feeding guidelines in the UK and has a particular focus on the practical implementation of safe formula preparation in real family settings — making his perspective directly relevant to the gap between knowing guidelines and applying them confidently at home.

For a broader overview of formula feeding — including choosing the right formula type, responsive bottle feeding, and recognising formula intolerance — visit our complete Formula Feeding guide in the Feeding and Nutrition section.

Frequently Asked Questions

Can I use bottled water to make formula?

Bottled water is not recommended as the default for making formula. Most bottled waters contain higher levels of certain minerals (particularly sodium and sulphate) than tap water and have not been designed for infant use. If tap water is unavailable — for example, when travelling — choose a still mineral water with sodium below 200mg/L and sulphate below 250mg/L, and still boil it before use. Your health visitor can advise for your specific circumstances.

How do I know if I am measuring the formula correctly?

Always use the scoop provided in the tin you are currently using — scoop sizes vary between brands and even between different formula products from the same brand. Level the scoop with a clean, dry, flat edge (a knife works well) rather than tapping it against the side. Heaped, packed, or tapped-down scoops deliver more powder than intended. If in doubt, your health visitor can check your preparation technique.

My baby seems to prefer cold formula — is that okay?

Yes. Formula does not need to be warm to be nutritionally appropriate or safe. Some babies accept cold or room-temperature formula readily, which simplifies preparation significantly. As long as the formula was prepared safely with water above 70°C and stored correctly, the serving temperature is a matter of preference. If your baby accepts cold or room-temperature formula, you do not need to warm it.

How long can a prepared bottle sit out at room temperature before I need to discard it?

A prepared feed that has not been offered to the baby should be discarded after two hours at room temperature. Once a baby has fed from a bottle, discard any remaining formula immediately after the feed — do not save it. The combination of room temperature and bacteria from the baby’s mouth creates conditions for rapid bacterial growth.

Is it safe to use a formula from a tin that has been open for a while?

Most formula tins should be used within four weeks of opening. Check the specific guidance on your tin, which will give the recommended use-by period once opened. Store opened tins in a cool, dry place (not the fridge) with the lid sealed. Never use formula beyond the printed use-by date on the tin, and never use a tin that has been damaged, dented significantly, or stored in unusual conditions.

Key Takeaways

• Water must still be above 70°C when added to formula powder — use freshly boiled water within 30 minutes, never pre-cooled or filtered water.
• Always add water to the bottle first, then add the correct number of level (not heaped) scoops of powder.
• Cool prepared feeds rapidly under cold water and test temperature on the inside of the wrist before feeding.
• Store prepared formula in the back of the fridge and use within 24 hours — never freeze, never reheat in a microwave.
• Discard any formula left in a bottle after a feed immediately — do not save partially consumed feeds.
• Ready-to-feed liquid formula eliminates preparation risk and is a practical choice for the early weeks or when safe preparation is not possible.

Formula preparation is one of those tasks that feels complicated until it becomes routine — and then it becomes as automatic as any other part of feeding. The steps above are worth reading carefully once, practising before the baby arrives if possible, and then trusting. The guidelines exist because they work. Following them consistently gives your baby every protection they need.