ABV — alcohol by volume — is just the difference between two gravity readings, scaled by a constant. Take a reading before fermentation (original gravity) and another when it's finished (final gravity), and the math falls right out. Here's exactly how to do it by hand.
Open BrewLog →Subtract your final gravity from your original gravity, then multiply the gravity points by 131.25. That single constant is the homebrew standard and is what most brewing apps report.
You measured OG = 1.050 and after two stable readings FG = 1.010.
So your pale ale is a sessionable 5.25%.
A bigger beer: OG = 1.066, FG = 1.012.
Notice the only thing that changes is the gravity drop — the wider the gap between OG and FG, the more sugar the yeast converted to alcohol.
The simple ×131.25 formula slightly under-reports as ABV climbs past about 7–8%. For big beers, brewers often use the Hall alternate equation:
Run example 2 through it and you get roughly 7.2%, a touch higher than the simple formula. For everyday session and standard-strength beer, the difference is small enough that ×131.25 is perfectly fine.
Gravity is measured against water at 1.000; sugar makes wort denser, so unfermented wort reads above 1.000 and drops as yeast eats the sugar. Remember to temperature-correct hydrometer readings, since a warm sample reads artificially low. Calibration matters: an OG that's off by 0.002 shifts your ABV by about a quarter of a percent.
BrewLog does all of this automatically — log your OG and each dated FG reading and it computes live ABV and attenuation, so you never have to reach for a calculator. It's free, offline, and private to your browser.
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