ABV from gravity readings
Two hydrometer readings bracket the fermentation — OG before, FG after, ABV between them.
Tools used in this guide
The job
Original gravity measures sugar before yeast; final gravity what's left. Their difference maps to alcohol. Homebrew tradition uses (OG−FG)×131.25; a longer formula corrects at high gravities.
Two gravities
Enter OG (say 1.055) and FG (say 1.012). The tool shows both ABV formulas plus apparent attenuation — how much of the sugar fermented away.
Worked example
1.055 → 1.012: Δ = 0.043 → ≈ 5.6 % ABV standard, ≈ 5.8 % alternate, ~78 % attenuation — a solid dry ale.
Where it stops
Both formulas approximate; for >7 % beers the alternate is closer to lab values. Hydrometer reads at 20 °C/60 °F assumed — correct hot samples first.