Hydrometer Temperature Correction.
Hydrometers are calibrated at a fixed temperature — typically 20 °C / 68 °F. A reading taken above or below that drifts predictably. Enter your measured SG and the actual sample temperature, and this calculator returns the corrected value using the Lyons 1992 polynomial.
Inputs
Temperatures can be entered in either unit.
How this is calculated.
Show the formula and citation
The Lyons polynomial fits a density correction factor as a cubic in temperature (°F):
Inputs are converted from °C to °F internally; you only see the corrected SG and the delta. If sample temperature equals calibration temperature, the correction is zero.
Worked example.
Measured SG 1.050 · Sample 30 °C / 86 °F · Calibration 20 °C / 68 °F
factor(86 °F) = 1.00332 · factor(68 °F) = 1.00084
Corrected = 1.050 × 1.00332 / 1.00084 = 1.0526
The correction was +0.0026 — small, but enough to bump apparent ABV by about 0.15% if you trusted the uncorrected number.
Common mistakes.
- Reading a sample straight from the kettle. Hot wort is volatile and reads inaccurately even after correction. Cool to under 40 °C before measuring.
- Mixing up "measured" and "corrected". Always log both values plus the sample temperature, so the correction can be re-applied or audited later.
- Assuming your hydrometer is calibrated at 20 °C. Some older hydrometers, especially second-hand ones, are calibrated at 15 °C or 60 °F. The printed scale will tell you.
- Applying this to a refractometer. Refractometers measure differently and need their own correction. Don't combine the two.
Related calculators.
Frequently asked.
What calibration temperature does my hydrometer use?
Most modern brewing hydrometers are calibrated at 20 °C / 68 °F. Some older designs use 15 °C / 59 °F or 15.6 °C / 60 °F. Check the printed scale or the box — if it isn't marked, 20 °C is the safer assumption.
How big is the correction?
Small. A 10 °C difference from calibration produces about a 0.001 SG change. The correction matters most when measuring hot wort during brew day, where the difference can reach 0.005 SG or more.
Should I correct a refractometer reading the same way?
No. Refractometers compensate for temperature differently (some have ATC built in) and need a different correction approach. They also drift in the presence of alcohol — a separate calculator handles that case.
Can I just wait for the sample to cool?
Yes — cooling the sample to calibration temperature is the most accurate approach. Use this calculator when waiting isn't practical, for example when checking mash gravity mid-brew.