Refractometer Correction Calculator
Correct refractometer Brix readings for alcohol to estimate true final gravity and ABV.
Reading before pitching
Reading now / at the end
FG ≈ -
Original gravity-
Estimated ABV-
How it works
We divide both readings by the wort correction factor, then apply Sean Terrill's cubic model, which estimates true final gravity from the initial and current refractometer Brix in the presence of alcohol.
FG = 1.0000 − 0.0044993·OB + 0.011774·FB + 0.00027581·OB² − 0.0012717·FB² − …
Source: Sean Terrill, refractometer FG cubic model.
Refractometer FG models are approximate. For final packaging decisions, confirm with a hydrometer.
Frequently asked questions
- Why are refractometer readings wrong during fermentation?
- Refractometers measure light refraction, which alcohol changes. Once fermentation produces alcohol, a raw Brix reading no longer reflects true gravity. This tool applies a correction model to recover final gravity and ABV from the initial and current Brix readings.
- What is the wort correction factor (WCF)?
- Refractometers are calibrated for sucrose, but wort refracts slightly differently, so readings run a touch high. Dividing by a wort correction factor (typically 1.04, but check yours against a hydrometer) aligns the refractometer to actual wort gravity.