Refractometer Calculator (Brix to Gravity)
Brix to gravity for wort with your correction factor, and final gravity and ABV once alcohol is present, by Sean Terrill’s correction.
Original gravity from a wort reading and your wort correction factor, and once fermentation has started the true final gravity by Terrill’s cubic, the ABV and attenuation — with what the uncorrected reading would have said.
Example: 13 °Bx at the start and 6.5 °Bx now, correction factor 1.04: OG 1.050 and FG 1.012 — not the 1.025 the raw reading suggests — for 5.15 % ABV.
Alcohol bends
the light.
What the wort correction factor does, why a refractometer misreads fermenting beer, and how Terrill’s formula recovers the gravity.
Wort
A refractometer is calibrated for sucrose in water. Wort contains other sugars and reads slightly high, so the reading is divided by a wort correction factor found by comparing your refractometer with a hydrometer on the same wort. The corrected Brix is read as °Plato and converted to gravity with the cubic fit to the ASBC extract table.
After fermentation starts
Alcohol raises the refractive index more than it changes density, so a Brix reading of fermenting beer is no longer a sugar reading: 6.5 °Bx looks like 1.025, when the beer is really about 1.012. The final gravity has to be worked out from both the original and the current reading.
Terrill’s cubic
Sean Terrill fitted a cubic in the corrected original (OB) and current (FB) Brix to paired refractometer and hydrometer readings: FG = 1 − 0.0044993·OB + 0.011774·FB + 0.00027581·OB² − 0.0012717·FB² − 0.0000072800·OB³ + 0.000063293·FB³. ABV then follows from Hall’s formula.
What the numbers do not settle
The fit comes from beer-strength worts; wine and mead musts, and very strong worts, are outside what it was fitted to. Read at the temperature your refractometer compensates for. Inputs stay in your browser; the same anonymous usage counts as the rest of the site apply.
SOURCES
- Sean Terrill, “Refractometer Estimates of Final Gravity” (seanterrill.com, 2010–2011)
- ASBC Methods of Analysis — extract (°Plato) table, as its cubic fit in specific gravity
- Michael L. Hall, “Brew by the Numbers — Add Up What’s in Your Beer”, Zymurgy, Summer 1995
Last reviewed 24 September 2026. How results are checked: How we verify.