Beer ABV Calculator (OG/FG)
Calculate the alcohol by volume, apparent attenuation and gravity drop of a homebrew from its original and final gravity.
How it works
ABV ≈ (OG − FG) × 131.25 · Apparent attenuation = (OG − FG) ÷ (OG − 1) × 100
Fermentation turns sugar into alcohol and CO₂, so the drop in specific gravity from OG to FG measures how much sugar was consumed. The classic homebrew shortcut multiplies that drop by 131.25 to get ABV, which is accurate up to roughly 6–7%; a refined formula (shown as the alternate) tracks better at higher gravities. Apparent attenuation reports what fraction of the available sugar the yeast fermented.
Worked example
A beer that starts at 1.050 and finishes at 1.010 dropped 40 gravity points. ABV ≈ (1.050 − 1.010) × 131.25 = 5.25%, and attenuation = 0.040 ÷ 0.050 = 80%.
Frequently asked questions
Why are there two ABV numbers?
The simple ×131.25 formula is easy and accurate for typical session and pale-ale strengths. The alternate formula corrects for the non-linear relationship at higher gravities, so it is closer for strong beers above about 7% ABV.
What is apparent attenuation?
It is the percentage of the starting gravity points the yeast fermented out. 75–80% is typical for ale yeast; higher attenuation means a drier, thinner beer, lower means more residual sweetness and body.
Do I need to correct gravity for temperature?
Yes. Hydrometers are calibrated at a set temperature (often 20°C / 68°F). Readings taken warm or cold should be temperature-corrected before entering them, or your ABV will be slightly off.
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Educational estimate for homebrewing. Please drink responsibly.