Kettle Boil-Off Rate and Post-Boil Gravity
Carries extract through the boil in point-gallons to give the post-boil volume and gravity, hot and cooled.
Example
You enter
- Pre-boil volume (gal) 350
- Pre-boil gravity 1.049
- Boil-off rate (% per hour) 8
- Boil duration (hours) 1
- Cooling shrinkage (%) 4
- Harder boil to compare (% per hour) 12
You get
- Extract, constant throughout 17,150 point-gallons
- Evaporated (gal) 28
- Hot, after the boil 322
- Into the fermenter 309.12
- Cooled og points 55.4801
- Hard cooled volume (gal) 295.68
- Hard og points 58.0019
Checked against published worked examples from: Brew Your Own.
Details, formula, and sources
ScopeThe worked 350 gal at 1.049 goes into the fermenter at 309.1 gal and 1.055. A 12% boil gives 1.058 and 13.4 gal short -- two errors that hide each other when topped up to volume.
Formula or tablepoint-gallons = points x volume, constant; evaporated = V x rate x hours; cooled = hot x (1 - shrinkage); gravity = point-gallons / volume.
Source and editionConservation of extract, with cooling shrinkage about 4%. The kettle's own measured boil-off rate governs.
Free accessPublic arithmetic.
What governsEstimate. AHJ and licensed professional govern.
API field namespreboil_volume_gal, preboil_gravity, boiloff_pct_per_hour, boil_hours, shrinkage_pct, hard_boiloff_pct_per_hour, extract_point_gallons, evaporated_gal, hot_volume_gal, cooled_volume_gal, cooled_og_points, hard_cooled_volume_gal, hard_og_points
- Boil-off a fraction of the pre-boil volume per hour, the brewhouse conventionspec-v1782