Formula
Michael L. Hall’s 1995 cubic fits the density of water. The correction factor at a Fahrenheit temperature is CF(T) = 1.00130346 − 1.34722124×10⁻⁴ T + 2.04052596×10⁻⁶ T² − 2.32820948×10⁻⁹ T³. Corrected gravity is the reading times CF(sample) ÷ CF(calibration). Dividing by the calibration factor is what keeps a 60°F reading on a 60°F hydrometer at the number you measured. Multiplying by CF alone, which is the form Hall printed for a 60°F instrument, shifts a 60°F reading by about 0.07 points. This page uses the ratio.
SG = measured × CF(sample) / CF(calibration)
°Bx = (SG − 1) × 258.6 / (1 + (SG − 1) × 227.1 / 258.2)
Brix to specific gravity is the ASBC-style conversion already on the ABV page, and the line above is its inverse. A 1.050 round-trips through 12.385 Brix. The refractometer original and final gravities are Sean Terrill’s 2011 cubic, called unchanged from the ABV calculator. The simple ABV under the result is (OG − FG) × 131.25, so you can see the alcohol without leaving the page. The fit is meant for roughly 32–212°F.
Worked examples
1.050 at 75°F, calibrated at 60°F. Corrected gravity is 1.0517. The same reading at 60°F stays 1.050.
Specific gravity 1.050 as Brix. 12.39 °Bx. Converting 12.385 Brix back returns 1.050.
Refractometer 12 Brix original and 6 Brix final, wort correction 1.04. The same Terrill result as the ABV page: original gravity near 1.046 and an alcohol-corrected final gravity near 1.011.
Reference
CF(60°F) is 1.000063. The ratio CF(60)/CF(60) is 1, so the reading is unchanged. Hall’s multiply-by-CF form would have made it 1.05007.
Questions
What is 1.050 at 75°F on a hydrometer calibrated at 60°F?
1.0517. The correction is Hall’s 1995 cubic, used as a ratio so a reading taken at the calibration temperature does not move. At 60°F the same 1.050 stays 1.050.
What Brix is a specific gravity of 1.050?
12.39 °Bx, from the inverse of the ASBC-style conversion on the ABV page. Converting that Brix back returns 1.050.
Can a refractometer read final gravity?
Not the raw number. Alcohol pulls the reading. This page uses Sean Terrill’s 2011 cubic, the same function as the ABV calculator, with a wort correction that defaults to 1.04. A hydrometer is still the direct measurement.
Sources
- Michael L. Hall, “Brew by the Numbers,” Zymurgy 18(2), Summer 1995, pp. 54–61. The cubic is his fit. This page applies it as CF(sample)/CF(calibration) so any calibration temperature is an identity at that temperature.
- Brix ↔ SG is the ASBC-style conversion shared with the ABV page: SG = 1 + Bx / (258.6 − (Bx / 258.2) × 227.1).
- Sean Terrill, refractometer cubic, 2011, called unchanged from the ABV calculator. Wort correction defaults to 1.04.