Formula
The simple brewer’s rule multiplies the gravity drop by 131.25. The alternate formula used by a lot of brewing calculators bends that line. Both take specific gravity, not Brix.
simple = (OG − FG) × 131.25
alternate = (76.08 × (OG − FG) / (1.775 − OG)) × (FG / 0.794)
proof = 2 × ABV
Brix converts to gravity with 1 + brix / (258.6 − (brix / 258.2) × 227.1). Finished beer needs more than that, because alcohol is in the sample. Sean Terrill’s 2011 cubic, after a wort correction factor, is FG = 1 − 0.0044993·OE + 0.0117741·FE + 0.000275806·OE² − 0.00127169·FE² − 0.00000727999·OE³ + 0.000063293·FE³, where OE and FE are the Brix readings divided by the correction factor. The coefficients were not re-fetched from Terrill’s page for this batch. A hydrometer is the measurement the simple formula expects.
Worked examples
OG 1.050, FG 1.010. Simple ABV is 0.040 × 131.25 = 5.25%. Alternate is (76.08 × 0.040 / 1.725) × (1.010 / 0.794) = 5.34%. US proof of the simple result is 10.5.
Refractometer 12°Bx starting, 6°Bx finishing, correction 1.04. Original gravity comes back as 1.046 and final gravity as 1.011. Simple ABV on those two is 4.6%.
OG 1.080 to FG 1.018. The simple and alternate results separate. Use the alternate when you are comparing with a calculator that prints that formula, and say which one you used.
Reference
Simple ABV, (OG − FG) × 131.25, for a final gravity of 1.010.
| OG | Simple | Alternate |
|---|---|---|
| 1.040 → 1.010 | 3.94% | 3.95% |
| 1.050 → 1.010 | 5.25% | 5.34% |
| 1.060 → 1.010 | 6.56% | 6.77% |
| 1.070 → 1.010 | 7.88% | 8.24% |
| 1.080 → 1.010 | 9.19% | 9.75% |
Questions
What ABV is 1.050 down to 1.010?
5.25% by the simple rule, (1.050 − 1.010) × 131.25. The alternate formula gives 5.34%. The two stay close on a beer this size and pull apart as the original gravity climbs.
Why are there two formulas?
The simple rule is a straight line. The alternate formula, (76.08 × (OG − FG) / (1.775 − OG)) × (FG / 0.794), bends with gravity. Report the one you mean. This page shows both.
Can I use a refractometer on finished beer?
Not the raw Brix. Alcohol pulls the reading. This page applies Sean Terrill’s 2011 cubic after dividing the Brix by a wort correction factor, default 1.04. A 12 Brix starting reading and a 6 Brix finishing reading come back near 1.046 and 1.011. A hydrometer is still the direct measurement of final gravity.
Sources
- Simple ABV: (OG − FG) × 131.25.
- Alternate: (76.08 × (OG − FG) / (1.775 − OG)) × (FG / 0.794).
- Brix to SG: 1 + brix / (258.6 − (brix / 258.2) × 227.1).
- Sean Terrill, refractometer FG cubic, 2011, with a wort correction factor defaulting to 1.04. Coefficients were not re-read from the original page in this pass.