Gravity & ABV
How your grain bill becomes alcohol
Gravity is the density of your wort. It tells you how much sugar is dissolved in it. Original gravity (OG) is measured before fermentation. Final gravity (FG) is what's left after yeast has eaten the fermentable sugars. The difference between them determines your ABV.
In the recipe builder, OG, FG, and ABV all update automatically as you adjust your grain bill, batch size, efficiency, or yeast selection. The whole pipeline is connected. Change one thing and everything recalculates.
Original Gravity
Every fermentable in your recipe has a PPG value: points per pound per gallon. It's a measure of how much sugar that ingredient contributes. 2-Row malt has a PPG of ~37, meaning 1 pound in 1 gallon gives a gravity of 1.037.
PPG = extract potential, W = weight (lbs), η = efficiency (100% for sugars/extracts, mash efficiency for grains), V = batch volume (gal).
Sugars (corn sugar, honey, candi sugar) and extracts (DME, LME) use 100% efficiency. They dissolve completely and bypass the mash. Only grains that go through the mash get your mash efficiency applied. This is the industry standard approach.
Final Gravity
FG depends on two things: which sugars are fermentable, and how thoroughly your yeast ferments them. Not all grain extract is fermentable. Crystal malts produce less fermentable sugar than base malt, and lactose isn't fermentable at all.
Per-ingredient fermentability
Each ingredient has a fermentability value from 0 to 1. Base malt is 1.0 (fully fermentable by the yeast's stated attenuation). Crystal malts range from 0.90 (light) down to 0.75 (dark), based on Briess maltster data. Lactose is 0. The gravity from each ingredient is split into fermentable and non-fermentable fractions.
Mash temperature effect
Mash temperature is the biggest lever on final gravity. Lower temps favor beta-amylase (more fermentable sugars, lower FG). Higher temps favor alpha-amylase (more dextrins, higher FG, fuller body). We offer three models for this. See the mash temperature page for the full story.
ABV
The industry standard approximation. Accurate to within 0.1% for beers under 1.080 OG.
The 131.25 constant comes from the well-established relationship between gravity drop and ethanol production. For the full breakdown, see the ABV calculator page.
See all the numbers come together in real time.
See this in the recipe builder