Beer ABV Calculator

Beer is the easy case for a hydrometer and the awkward case for a refractometer. Wort before pitching is a sugar solution, so a refractometer reads it accurately. Finished beer is a mixture of ethanol, unfermented dextrins and water, and ethanol bends light more strongly than the maltose the instrument was calibrated on, so a raw final Brix reading comes out high. The refractometer mode corrects that reading using your original Brix, which is why it asks for both. For a 1.050 ale the correction moves the final gravity by a few thousandths, for a 1.090 imperial stout it moves the ABV by more than half a percent.

Hydrometer readings need a different correction. Wort is usually read warm after chilling, and every 5°C above the 20°C calibration point adds roughly 0.001 to the true gravity. Turn on temperature correction and enter the sample temperature for each reading rather than the fermenter temperature.

The full Terrill correction and the hydrometer temperature polynomial, with their references, are on the general fermentation ABV calculator. For the reasoning behind temperature correction see hydrometer temperature correction.

Worked example. A pale ale starts at 1.052 and finishes at 1.011, both read at 20°C. The Balling formula this page runs gives 76.08 × (1.052 − 1.011) ÷ (1.775 − 1.052) = 4.31 percent alcohol by weight, and multiplying by 1.011 ÷ 0.794 converts that to 5.5 percent ABV. The quick (OG − FG) × 131.25 estimate gives 5.4 percent, close enough at this strength, but the gap widens to a full percent or more above 1.090. Apparent attenuation is (0.052 − 0.011) ÷ 0.052 = 79 percent, squarely in the healthy range for an American ale strain.

Malted barley does not convert entirely into fermentable sugar. A portion of the starch becomes dextrins, longer sugar chains that brewing yeast cannot metabolise. Those dextrins stay in solution after fermentation ends, which is why a finished beer almost never drops below about 1.008 even with a fully healthy fermentation.

The split between fermentable sugar and dextrin is set in the mash. A lower mash temperature around 63 to 65°C favours beta amylase and produces a more fermentable wort, giving a drier beer with a lower final gravity. A higher mash near 68 to 72°C favours alpha amylase, leaves more dextrin behind, and finishes higher with more body.

Typical homebrew range: original gravity 1.040 to 1.060, final gravity 1.008 to 1.015. If your final gravity is below 1.005, either the mash ran very thin or something other than brewing yeast is fermenting the beer.

Apparent attenuation is the share of the original extract that fermented, read straight from the two gravities: (OG minus FG) divided by (OG minus 1.000). For beer it is the number that tells you whether the mash and the yeast did what you planned.

Clean ale fermentations land between 72 and 82 percent. English ale strains sit at the low end and leave body, American and Belgian strains at the high end, and saison yeasts can pass 90. Lagers run 75 to 85. If you measure under 65 percent with a strain rated higher, fermentation is either still going or has stalled: take a second reading two days later before touching it. Beer never reaches 100 percent, because the dextrins left by the mash are not fermentable by brewing yeast, so a wine or cider attenuation figure is not a fair comparison.