How do you convert a refractometer reading to specific gravity?
For unfermented wort, two steps. A refractometer is calibrated for sucrose in water, and malt sugars bend light a little more than sucrose does — so the °Brix it shows is high by the wort correction factor, about 1.04. Divide by it to get °Plato, then convert Plato to gravity as usual. A reading of 12 °Bx is 11.5 °P of actual wort, or 1.046. Skip the factor and you'd write down 1.048 — two points high, every batch, which is exactly the size of "I always seem to miss my OG".
°P = °Bx ÷ WCF · SG = 1 + °P ÷ (258.6 − °P ÷ 258.2 × 227.1)
Why is my refractometer wrong after fermentation?
Because it doesn't measure sugar. It measures how much the liquid bends light, and reports that as if the only thing dissolved were sucrose. Once yeast has turned sugar into ethanol, the alcohol bends light too — so a fermenting beer reads far higher than its true gravity, and the instrument can't tell you how much of the reading is sugar and how much is alcohol. You break the tie with the original Brix: knowing where the wort started, a fitted formula can back out how much alcohol must be present and what the real gravity is.
Two such fits are in common use, both from Sean Terrill's 2011 work on hydrometer-measured beers, later refit by Novotny (Zymurgy, 2017):
Cubic: FG = 1 − 0.0044993·OB + 0.0117741·FB + 0.000275806·OB² − 0.00127169·FB² − 0.00000728·OB³ + 0.0000632929·FB³
Linear: FG = 1 − 0.000856829·OB + 0.00349412·FB
OB and FB are the original and current Brix, each already divided by the WCF. (One widely copied version of the cubic prints the OB³ coefficient ten times too large — check yours if you copy it.)
For a wort that read 12 °Bx and now reads 6: the cubic says 1.011, the linear 1.010, and the uncorrected reading would have you believe 1.023 — eleven points too high, and an ABV of 3.1% instead of the real 4.6%. Across the range:
| OB → FB (°Bx) | OG | Cubic | Linear | Uncorrected |
|---|---|---|---|---|
| 10 → 4.5 | 1.038 | 1.008 | 1.007 | 1.017 |
| 12 → 6 | 1.046 | 1.011 | 1.010 | 1.023 |
| 13 → 6.5 | 1.050 | 1.012 | 1.011 | 1.025 |
| 15 → 7.5 | 1.059 | 1.013 | 1.013 | 1.029 |
| 18 → 9 | 1.071 | 1.015 | 1.015 | 1.034 |
| 20 → 10 | 1.080 | 1.016 | 1.017 | 1.038 |
| 24 → 12 | 1.097 | 1.017 | 1.021 | 1.046 |
WCF 1.04 throughout; from this page's engine. Shaded rows: the two fits disagree by more than 1.5 points — big beers, where neither has much data behind it.
They agree to about a point through ordinary beers, so it rarely matters; the calculator leads with the cubic because it's the one the big tools use and it tracks hydrometer data a shade better in Terrill's own testing. Both are fits — regressions over someone else's batches, not physics — and both drift on very strong or very sweet beers. Treat the answer as ±0.002 and, if the number matters (a competition entry, a stuck-fermentation call), confirm the FG with a hydrometer. The refractometer's superpower is the OG and the daily "is it still dropping?" check from a few drops, not the final word.
How do you find your refractometer's wort correction factor?
Once, on any brew day: read the same unfermented, cooled wort with the refractometer and with a temperature-corrected hydrometer. Convert the hydrometer's SG to Plato; WCF = Brix ÷ Plato. Most instruments land between 1.02 and 1.06; write yours on the case and use it in every calculation from then on. The "Find my WCF" mode above does the arithmetic and hands the value back to the other two modes.
There are two separate corrections and people conflate them. First, the zero: a drop of distilled water at room temperature should read 0.0 °Bx — if not, turn the calibration screw. That fixes the instrument. Second, the WCF: even a perfectly zeroed refractometer reads wort ~4% high because wort isn't sucrose. That fixes the sample. The ATC ("automatic temperature compensation") printed on the box handles neither — it only stops a warm sample from reading differently than a cool one, and only within its stated range.
Refractometer or hydrometer?
Both, for different jobs. The refractometer needs three drops and thirty seconds — perfect for tracking the mash, checking pre-boil gravity while you can still fix it, and confirming OG without wasting a trial jar of wort. The hydrometer needs a hundred milliliters and a cooled sample, but it reads a fermented beer directly, with no assumptions about where it started. So: refractometer for everything up to pitching, either for watching fermentation move, and a hydrometer for the FG you're going to write on the label. If you only own a refractometer, this calculator gets you close — but you'll never know quite how close.