Refractometer Correction Calculator: True FG and ABV from Brix During Fermentation
Correct a refractometer Brix reading taken during or after fermentation for the alcohol present. Start from the original Brix or a hydrometer OG, choose the Terrill cubic or linear correction, and get the true final gravity, ABV (standard and high-gravity formulas), ABW, apparent and real attenuation, your wort correction factor and a reading-by-reading table and chart.
How it works
OB = original Brix ÷ WCF (or °P of the hydrometer OG); FB = current Brix ÷ WCF; Terrill cubic: FG = 1 − 0.0044993·OB + 0.011774·FB + 0.00027581·OB² − 0.0012717·FB² − 0.00000728·OB³ + 0.000063293·FB³; Terrill linear: FG = 1 − 0.00085683·OB + 0.0034941·FB; OG = 1 + OB ÷ (258.6 − 227.1·OB ÷ 258.2); ABV = (OG − FG) × 131.25, or alternate 76.08·(OG − FG) ÷ (1.775 − OG) × FG ÷ 0.794
A refractometer needs one drop and no temperature fuss, which makes it handy for tracking a fermentation, but alcohol bends light too. Once yeast turns sugar into ethanol the Brix reading sits well above the true sugar content, so a raw reading overstates the final gravity and understates the alcohol. Sean Terrill fitted cubic and linear equations to several hundred paired hydrometer and refractometer readings of fermented beer; given the original and current Brix (both divided by the wort correction factor) they return the true final gravity within about a gravity point for normal-strength beer. The cubic is what most brewing software uses; the linear fit is a little less precise but tracks partly fermented beer more evenly. The wort correction factor (WCF) exists because a refractometer is scaled for pure sucrose and wort reads about 4% high. To find yours, read the same unfermented wort with the refractometer and a hydrometer; the calculator divides the Brix by the hydrometer’s °Plato when you enter that hydrometer OG under More options. If you measured OG with a hydrometer only, switch the first field to Hydrometer: the OG is then used directly and only the current reading is divided by the WCF. ABV comes from the standard (OG − FG) × 131.25 formula or the alternate formula that holds better for strong beer, and real extract uses the Balling relation RE = 0.1808·OE + 0.8192·AE. Results are for malt wort; mead, wine and cider ferment different sugars and the fit is less certain there.
Worked example
Original 13.5 °Bx, now reading 6.8 °Bx, WCF 1.04, Terrill cubic: OB = 13.5 ÷ 1.04 = 12.98 °P, so OG = 1.053 (1.0525); FB = 6.8 ÷ 1.04 = 6.54. The cubic gives FG = 1.0125, shown as 1.012. ABV = (1.0525 − 1.0125) × 131.25 = 5.3%, apparent attenuation 76%, real extract 5.0 °P and real attenuation 62%. Read naively, 6.54 °Bx would look like FG 1.026, 13 points too high. The Terrill linear formula gives 1.012 as well.
Sources: Lacada Brewery: refractometer calculator (Terrill cubic coefficients) · Brewer’s Friend: refractometer calculator · Brewfather: refractometer tool documentation (WCF 1.04, range 1.02–1.06) · HomeBrewTalk: refractometer calculators and final SG (Terrill linear and cubic results)
Frequently asked questions
How do I find my wort correction factor?
Measure the same unfermented wort with a hydrometer and the refractometer, then divide the Brix by the hydrometer’s °Plato. Enter the hydrometer OG under More options and the calculator does it for you. Most instruments land between 1.02 and 1.06; average a few brew days.
Is a refractometer accurate enough for the final ABV?
With a good WCF the corrected FG is usually within about a point of a hydrometer, about ±0.2–0.3% ABV for a normal beer. For labels, competitions or beers over about 1.080 OG, confirm the FG with a hydrometer.
Cubic or linear Terrill formula?
The cubic is the standard in most brewing software and fits fully fermented beer well. The linear one is a little less precise but behaves more evenly part-way through fermentation. The calculator shows both so you can see how far apart they are.
Why does my refractometer read high after fermentation starts?
Ethanol has a higher refractive index than water, so the instrument counts it as if it were sugar. Any reading taken after pitching must be corrected with the original gravity.
Can I use this for wine, cider or mead?
You can, with a WCF of 1.00 for fruit juice, but Terrill’s data were for malt wort. Treat the result as an estimate and check the end of fermentation with a hydrometer.
What if I only took a hydrometer reading at the start?
Choose Hydrometer for the original gravity and enter the OG. The calculator converts it to °Plato for the correction and only divides the current Brix by the WCF.
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This calculator is for educational purposes. Double-check important results before acting on them.