I forgot to measure how much wort I had preboil. I did take a gravity reading preboil.

Can I deduce preboil volume with post boil volume and pre & post boil gravity?

  • Is there any practical purpose of knowing pre-boil volume?
    – Mołot
    Commented Apr 17, 2016 at 15:12
  • 1
    Knowing boil off rate of my gear
    – Mild Fuzz
    Commented Apr 17, 2016 at 15:13

2 Answers 2


An answer offered from another forum:

It can be derived as a function of the difference in gravity

Pre boil Volume  = Post Boil Volume * (Post Gravity Points / Pre Gravity Point)

In this instance, pre gravity is 1.021, post is 1.030. Post boil volume is 23.7 litres

Pre Boil Volume = 23.7 * (30 / 21)

Pre Boil Volume = 33.9 litres
  • Were you intending to do such a vigorous boil (i.e. to increase the gravity)? 30+% evaporation over the course of a normal boil seems extreme for a batch that size. Commented Apr 18, 2016 at 16:52

If you know your exact boil of rate you can calculate it from pre boil gravity (providing the sample was well mixed), OG, and post boil volume.

However a record of pre boil volume is really only useful to determining boil off rate and mash effciency. Which are useful numbers to have for future batches but play little role in your current beer.

Your OG and post fermentation volume will give you the more useful number for getting your brewhouse efffeciency.

An easier way to get your pre boil volume rather than doing the math estimating boil off would be to fill your kettle with your known post boil volume and then record the volume it takes to get to the level of Wort you had before you started your boil.

  • You hit the nail on the head with the second para, it's this segment of my process I am trying to nail on
    – Mild Fuzz
    Commented Apr 17, 2016 at 17:41

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