Homebrew Boil-Off Volume Calculator
Estimate how much wort you lose to evaporation during the boil.
Enter pre-boil volume, boil time, and kettle diameter to hit your target batch size.
Why Boil-Off Matters
Pre-boil and post-boil volumes in homebrewing typically differ by 1 to 2 gallons on a standard 5-gallon batch. If you do not account for evaporation, you will either come up short on volume (concentrated wort, higher-than-expected OG) or overshoot your target by diluting too much to hit your batch size. Both outcomes throw off your final gravity.
What Drives Evaporation Rate
The surface area of your kettle is the primary factor, not total volume or how vigorously you boil alone. A wider, shallower kettle evaporates significantly more than a narrow, tall pot at the same volume, because evaporation scales with the area of the exposed liquid surface.
This calculator uses a surface-area model: evaporation rate = pi x r2 x 0.0067 gallons per hour per square inch of kettle opening. For a 12-inch diameter kettle, that works out to about 0.76 gal/hr. For a 14-inch kettle, about 1.03 gal/hr. These align closely with the commonly cited range of 0.75 to 1.25 gal/hr for typical homebrew setups.
Calibrating to Your Setup
If you track your actual boil-off across a few batches and find your real rate is different, adjust the kettle diameter up or down to match. For example, if your measured evaporation at 60 minutes is 1.1 gallons, enter a diameter of about 14.5 inches regardless of your actual kettle width.
Other Volume Losses
This calculator covers evaporation only. Budget an additional 0.5 to 0.75 gallons for trub and hop absorption loss from kettle to fermentor on a typical batch.