Fermentation 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 is approximately +0.001 SG per 5°C above calibration temperature. This calculator applies the polynomial correction formula for accurate results across a wide temperature range.

Apparent attenuation tells you what percentage of the available sugars have been fermented. A reading above 90% typically means fermentation is complete. Below 75% usually means fermentation is still active or has stalled.

For a sugar wash, expect attenuation of 90–98% with a healthy yeast. Grain or fruit mashes will attenuate less due to unfermentable dextrins. If attenuation is below 70% and the gravity has been stable for 3+ days, fermentation may have stalled — check temperature, pitch more yeast, or add nutrients.