Standard formula advice tends to say something like:

  1. Fill the kettle with at least 1 litre of fresh tap water (don’t use water that has been boiled before).
  2. Boil the water. Then leave the water to cool for no more than 30 minutes, so that it remains at a temperature of at least 70C.
  3. ...

(Source: NHS)

I am aware that you need to put the formula powder into the water when it is at least 70C so that any bacteria in the formula will be killed.

But is there a maximum temperature? Is it OK to mix the formula immediately after the kettle has boiled? Should I wait until the temperature has dropped below 90C? 80C?

I've failed to find any advice about this. (Also, why is it 1 litre when I'm only making 200ml of formula?)

  • Different strokes. Mayo Clinic in the US says you can use tap or bottled water--we used bottled water and didn't boil it ever. The litre recommendation might be assuming that you'll make more than one bottle and store the extras in the refrigerator.
    – mkennedy
    Commented Oct 31, 2016 at 14:35
  • 1
    @mkennedy Formula is prepared differently in different countries. In the UK the powder is not sterile so applying boiled water is necessary. In other countries the powder may be sterile and so tap water is fine. It's best to follow the instructions written on the specific formula to be safe. Also the 1L thing is because it's been worked out that 1L will cool to 70C after 30 minutes, which makes for easy to follow instructions. I've added an answer for that.
    – JBentley
    Commented May 3 at 14:23

2 Answers 2


The World Health Organization has created a handy pamphlet with fairly detailed instructions for preparing, storing and re-warming formula in care settings. There is no upper limit to the temperature when mixing the formula, but the water should be cool enough not to burn you. Boiling the water kills bacteria in the water, and a temperature of at least 70C will kill bacteria in the formula. The suggestion of a liter is for batch preparation. It is not recommended to pre-make more than one liter at a time.

Note that these instructions are for infants in "care settings" - so neo-nates that are either

  • pre-term
  • under 2.5 kg
  • less than 2 months old
  • immunocompromised

For infants in these categories, you can also buy sterile formula. Healthy, older babies are more resistant to bacteria.


The other answer mostly addressed the question so I won't attempt to re-answer those points. However, the reason that you boil 1 litre specifically isn't for batch preparation as suggested in that answer. It's because the target temperature is 70C and the average kettle will reach around 70C with 1L of water cooled for 30 minutes, which helps to make the instructions easy to understand and follow. If they didn't do that, they'd have to instruct you to actually measure the temperature of the water and monitor it over an extended time period until it reaches 70C which would be far more challenging.

  • 1
    Where have you seen this explained? It's a very interesting tidbit of information, but a source would make it valuable as opposed to merely interesting. (Live and learn!) Thanks. Commented May 4 at 13:50
  • @anongoodnurse It was in an antenatal course unfortunately, so there's no source for me to link to.
    – JBentley
    Commented May 4 at 17:42
  • 1
    Thanks for the reply. It would be nice to confirm. Commented May 4 at 19:16

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