About this tool
The Refractometer Final Gravity Correction calculator turns a refractometer reading taken during or after fermentation into a true specific gravity. Refractometers measure Brix by light refraction, but ethanol bends light differently from sugar, so once fermentation starts a raw refractometer reading overstates the remaining gravity. This tool removes that ethanol error using the widely used Sean Terrill correction, giving you a usable final gravity without needing a hydrometer sample. It runs entirely in your browser.
The method first applies a wort correction factor (WCF, default 1.04) to both readings: RI_i = OB / WCF for the original Brix and RI_f = FB / WCF for the current/final Brix. The Terrill cubic model then estimates true FG as 1.0000 − 0.0044993·RI_i + 0.0117741·RI_f + 0.000275806·RI_i² − 0.00127169·RI_f² − 0.00000727999·RI_i³ + 0.0000632929·RI_f³, and a linear variant is shown for comparison. Original gravity comes from RI_i via the Brix-to-SG relation, and ABV uses the common formula ABV = (76.08·(OG − FG)/(1.775 − OG))·(FG/0.794). Apparent attenuation is (OG − FG)/(OG − 1) × 100.
As a worked example, an original 12 °Bx that reads 6 °Bx at the end (WCF 1.04) corrects to an OG near 1.046 and a true FG around 1.011 by the cubic model, giving roughly 4.7% ABV. Set the WCF by calibrating your own refractometer against a hydrometer on a fresh wort (values of 1.02–1.06 are typical). The cubic model tends to read slightly high on very attenuated beers, which is why the linear estimate is provided alongside it.