Boil-Off Gravity Calculator

How it is calculated

The tool assumes that boiling removes water but no sugar, so the total sugar, measured as volume times gravity points, is constant. post-boil points = pre-boil points × pre-boil volume ÷ post-boil volume, and post-boil gravity = 1 + points ÷ 1000. Points are the digits after the decimal point of the specific gravity, so 1.042 is 42 points. Both volumes are in gallons. The points are rounded to a whole number, while the gravity is shown to three decimals and is calculated from the unrounded points. Volumes must be above zero and the points cannot be negative. The tool does not predict evaporation: you supply the expected post-boil volume yourself, for example from your kettle's measured boil-off rate.

Common mistakes

• Typing the full gravity, such as 1.042, into the points field. The tool then reports a final gravity of about 1.001, as if there were almost no sugar.
• Entering the volume that is lost to evaporation as the post-boil volume. The field is the volume left in the kettle, so 6.5 gal minus 1 gal lost is 5.5.
• Forgetting the volume lost to hop material and trub when you plan to measure what goes into the fermenter. The tool counts only the volume you enter.
• Ignoring that hot wort has a lower gravity reading than the same wort measured cooled. Take the pre-boil reading at the hydrometer's calibration temperature.
• Entering litres. Both volume fields are in gallons, so 25 litres must be converted to about 6.6 gallons.