As per this recipe for a Belgian Dark Strong Ale, I'm brewing a beer where it's suggested you take a portion of the pre-boil wort, and boil it down separately to create a syrup to add back to the main wort later.

Given this, how should I calculate my pre-boil volume?

Should I...

a) Use the same pre-boil volume as usual, but take a portion of this to boil into a syrup

b) Create a larger than usual pre-boil volume, to cater for the amount I take away

c) Just add some boiled top-up water at the end to compensate

d) Something else


I all-grain BIAB, and would need 9.75L (90 min) pre-boil wort for a desired post-boil vol of 6L

2 Answers 2


The recipe states:

Recipe Specifications

  • Batch Size: 5.50 gal
  • Boil Size: 7.75 gal
  • Estimated Pre-Boil Gravity: 1.065 SG
  • Estimated OG: 1.091 SG
  • Estimated FG: 1.006 SG
  • Estimated ABV: 11.4 %
  • Estimated Color: 36.8 SRM
  • Estimated IBU: 37.9 IBUs (Rager)
  • Brewhouse Efficiency: 75.00 %
  • Boil Time: 90 Minutes
  • Efficiency: 75.00

The optional step:

  1. Extract 1.0 gallon of wort at the beginning of clear runoff into a 3-5 gallon pot.

Therefore, letter a) is the most appropriate approach is you're strictly following the recipe. But, you'll have to scale your recipe down. 1gal on the optional step is too much for your batch size.

If you get a higher OG at the end of the boiling, then add water (using a dilution calculator) as needed.

  • Thanks, I ended up taking 1L out of my regular 9.75L pre-boil, not quite as much by ratio as 1 gallon of 7.75 - I didn't spot that on the 3rd page!
    – RYFN
    Jul 11, 2017 at 12:57

When I do a boil down, I add water at the end if needed. But I care more about boiling to gravity than volume. Keep in mind that your gravity will not change due to the boil down. You're boilng off water, not sugar, so when you return the boiled portion your gravity will be the same overall.

  • So potentially when you do a boil down, you're happy to end up with a smaller volume at then end, but with the correct gravity?
    – RYFN
    Jul 9, 2017 at 18:22
  • 1
    The volume may be smaller, but the gravity will increase. You can then add water at the end to get back to the volume and gravity you intended to have.
    – Denny Conn
    Jul 10, 2017 at 15:40

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