Beer ABV Calculator

A refractometer measures how much light bends when passing through a liquid. Before fermentation this works perfectly for measuring sugar concentration. After fermentation, alcohol in the wash bends light differently to water — making the reading appear lower than the actual gravity. If you use the raw refractometer reading as your FG you will overestimate your ABV.

This calculator applies the Terrill correction formula, which uses both the original Brix reading and the post-fermentation apparent Brix to recover the true final gravity:

Where Ri = initial Brix (OG reading) and Rf = apparent final Brix (post-fermentation refractometer reading). This is the most widely accepted correction formula for homebrewing and home distilling.

Most hydrometers are calibrated at 20°C (68°F). If your sample is warmer, the liquid is less dense and the hydrometer floats higher, giving a reading that is lower than the true gravity. The correction runs about +0.001 SG per 5°C just above calibration temperature and grows with the gap, reaching roughly +0.006 SG at 40°C. This calculator applies the polynomial correction formula for accurate results across a wide temperature range.

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 tells you what percentage of the available sugars have been fermented. A reading above 90% typically means fermentation is complete for a highly attenuative yeast. Below 65% usually means fermentation is still active or has stalled.

For an ale with a clean, healthy fermentation, expect apparent attenuation of 72 to 82% depending on yeast strain and mash composition. Lagers and Belgian styles can run higher. Because malt always leaves some dextrin behind, beer attenuation essentially never approaches 100%, unlike wine or cider.