Refractometer Correction.
Refractometers are calibrated for sucrose, but wort sugars refract differently — a small offset corrected by the Wort Correction Factor. Once fermentation begins, alcohol skews the reading further, and a different formula is needed. This calculator handles both cases: pre-fermentation Brix → true wort SG, and post-fermentation Brix → true final gravity and ABV using Sean Terrill's cubic fit.
Inputs
Pick the case that matches your reading.
When each correction applies.
Pre-fermentation: any time you take a refractometer reading before pitching yeast — mash sample, pre-boil gravity, post-boil OG check. Apply the WCF, then convert Brix to SG.
Post-fermentation: any refractometer reading taken after fermentation has produced alcohol. The raw reading is not a usable SG value; you need both the original Brix and the current Brix to back-solve true FG. The cubic formula is reliable in the 1.040–1.080 OG range and loses accuracy above 1.090, where a hydrometer FG reading is the better instrument.
How this is calculated.
Show the formulas and citations
Wort Correction Factor (pre-fermentation):
Terrill cubic (post-fermentation):
ABV (alternate formula, suitable for post-correction FG):
Worked examples.
Raw refractometer 16.5 °Brix · WCF 1.04
Corrected Brix = 16.5 / 1.04 = 15.87
True SG = 1 + (15.87 / (258.6 − 15.87/258.2 × 227.1)) = 1.0649
That's your OG — the same number a hydrometer at calibration temperature would have shown.
Original Brix 16.5 · Current Brix 7.5 · WCF 1.04
Corrected OB = 15.87 · Corrected FB = 7.21
Apply Terrill cubic → True FG 1.0115
ABV = (76.08 × (1.0649 − 1.0115) / (1.775 − 1.0649)) × (1.0115 / 0.794) = 7.27%
The raw post-fermentation refractometer reading of 7.5 °Brix would have suggested SG ≈ 1.029 — wildly wrong. The corrected FG of 1.0115 is realistic.
Common mistakes.
- Reading the raw post-fermentation Brix as a gravity value. A refractometer reading taken after fermentation begins is not a usable SG. Always apply the cubic correction with both OG and FG Brix, or use a hydrometer for FG.
- Forgetting the Wort Correction Factor on a pre-fermentation reading. Without WCF the refractometer overstates wort SG by ~0.002 — enough to plan a recipe to the wrong target gravity.
- Assuming WCF = 1.04 fits every refractometer. WCF varies by unit and by grist. Take parallel hydrometer + refractometer readings on one batch to dial in your specific WCF, then reuse that value.
- Trusting the cubic above OG 1.090. The Terrill cubic was trained on normal homebrew gravities. For barleywines, imperials, and wines with FG above 1.020 or OG above 1.090, take a hydrometer FG reading.
- Reading before the sample reaches prism temperature. Even with ATC, give the sample 20–30 seconds on the prism before reading. A reading taken immediately drifts as the sample equilibrates.
Related calculators.
Frequently asked.
Why do I need to correct a refractometer reading?
A refractometer measures the refractive index of a liquid, not its density. The default Brix scale is calibrated against pure sucrose solutions; wort sugars (mostly maltose, with some dextrins and proteins) refract slightly less per gram, so the raw reading overstates the true gravity. The Wort Correction Factor (WCF, typically 1.04) accounts for this. After fermentation begins the picture changes again — alcohol is more refractive than the water it replaced, so the raw Brix reading drifts even further from true gravity. A second formula is needed once alcohol is present.
What is the Wort Correction Factor (WCF)?
The WCF is a single number that converts raw refractometer Brix to true wort Brix: true_Brix = raw_Brix / WCF. The default of 1.04 is a good general value. The exact WCF for your refractometer can range from 1.02 to 1.06 depending on grist composition (more crystal and dextrin malts shift it higher). To dial in your specific number, take parallel hydrometer + refractometer readings on one batch of wort and back-calculate: WCF = raw_refractometer_Brix / hydrometer_implied_Brix. Use that WCF for future batches.
Why can I not use a refractometer for final gravity directly?
Alcohol has a higher refractive index per gram than water. Once fermentation begins, ethanol replaces sugar in the sample, and the refractometer reads the combined effect of dropping sugar and rising alcohol. The raw post-fermentation Brix reading is therefore meaningless as a direct gravity value — it overstates true SG by a substantial margin. Sean Terrill's cubic formula uses both the original Brix and the current Brix to back-solve true final gravity. This calculator implements that formula.
Which formula does this calculator use for post-fermentation correction?
Sean Terrill's cubic fit (2010), trained against 122 measured ferments and accurate to within 0.0015 SG across normal homebrew gravities (OG 1.040–1.080). The same fit is used by Brewer's Friend and BeerSmith. Above OG 1.090 the cubic extrapolates poorly and predicted final gravities can be off; for very high-gravity ferments (barleywines, big imperials, wines) a hydrometer FG reading is more reliable.
Should I use this for wine or mead?
For pre-fermentation, yes — but with a caveat. The Wort Correction Factor is calibrated for beer wort. Grape juice and honey-water musts behave closer to pure sucrose; you can use a WCF closer to 1.0 (or none at all) for mead and wine. For post-fermentation correction in mead and wine, the cubic formula is less validated — both have higher final ABVs that push outside the formula's trained range. A hydrometer is the more reliable instrument for FG on those crafts.
What about temperature correction?
Most modern refractometers have automatic temperature compensation (ATC) built into the prism — sample temperature is corrected internally before you read the scale, as long as you let the sample sit on the prism for 20–30 seconds first to reach prism temperature. If your refractometer is non-ATC, refer to its calibration temperature (typically 20 °C). This calculator assumes ATC; if yours is non-ATC, correct for temperature before entering the Brix reading.
How accurate is this versus a hydrometer?
Pre-fermentation, a properly WCF-corrected refractometer reading agrees with a hydrometer to within 0.001–0.002 SG. Post-fermentation, the cubic formula is accurate to about 0.0015 SG across normal gravities — close enough for most homebrew decision-making, but a hydrometer is the more direct measurement for ABV calculations where every 0.001 SG counts.
How do I calibrate my refractometer?
Place 2–3 drops of distilled water on the prism, close the cover, wait 20 seconds, and read the scale. It should read exactly 0.0 Brix. If it doesn't, use the small adjustment screw (usually under a cap on the body) to zero it. Recalibrate before each brew day, and whenever the unit has been transported or stored in a temperature-different location.