Pre-boil gravity??

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paanderson86

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I’m curious how many points I will gain from pre-boil to post-boil? For instance, if my target OG is 1.060 at 5.5 gallons, what should my gravity be at 7.0 gallons before starting?
 
It's a matter of the law of levers, and dilution math works the same way:

V1 = 7 gal
PG = ?

V2 = 5 gal
OG = 1.060 (but really we will use 0.060 since we care about the gravity points themselves)

PG * V1 = OG * V2

PG = OG * (V2/V1)

PG = 0.060 * (5/7) = 0.043

So, PG = 1.043 at 7.0 gal
 
It's important to subtract 1 from the SG's when doing the calculations. The rigorous way to write the equations are:

(Pre-Boil SG - 1) * Pre-Boil Volume = (Post-Boil SG - 1) * Post-Boil Volume

Pre-Boil SG = 1 + (Post-Boil SG - 1) * Post-Boil Volume / Pre-Boil Volume
Using OP numbers:

Pre-Boil SG = 1 + (1.060 - 1) * 5.5 gal / 7.0 gal
Pre-Boil SG = 1 + 0.060 * 5.5 / 7.0
Pre-Boil SG = 1.047
Brew on :mug:
 

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