Once you know your original and final gravity, your beer's alcohol content is simple arithmetic. BrewLog does the math automatically as you log readings — but here's exactly how it works so you can check it by hand.
Open BrewLog →Take your original gravity (OG, e.g. 1.050), subtract the final gravity (FG, e.g. 1.010), and multiply by 131.25. That example gives (1.050 − 1.010) × 131.25 = 5.25% ABV. The constant 131.25 is the standard homebrew approximation; it's accurate for typical beer strengths and is the figure most brewing software reports.
Attenuation tells you how much of the available sugar the yeast actually fermented. For the same example, (0.050 − 0.010) ÷ 0.050 = 80%. Most ale yeasts land between 70% and 85%; a number far below the strain's spec is a clue your fermentation stalled or your mash was too cool.
A calculator gives you one answer and forgets it. BrewLog keeps every dated reading per batch, recomputes current ABV and attenuation live from your latest measurement, and shows whether gravity has gone stable. You build a searchable history of every brew — free, offline, stored only in your browser. Pro adds CSV export and trend charts.
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