Hydrometer Temperature Correction.
A hydrometer is calibrated at a fixed temperature — typically 20 °C / 68 °F. A specific gravity reading taken above or below that drifts predictably, because the sample's density shifts with temperature. Enter the measured SG and the actual sample temperature in °C or °F, and this calculator returns the corrected value using the Lyons (1992) polynomial — the same correction factor used by Brewer's Friend, BeerSmith, and Brewfather.
Inputs
Temperatures can be entered in either unit.
When to correct, when to skip.
Within 2–3 °C of your hydrometer's calibration temperature, the specific-gravity correction is under 0.0005 SG and you can usually ignore it. Outside that window the correction matters, and there are three scenarios where it matters most:
Mash and pre-boil samples are typically pulled at 60–70 °C and read after a quick cool — a 30 °C offset from calibration produces roughly a +0.005 SG correction. If you treat the uncorrected reading as your true OG, your apparent ABV will be off by ~0.3% and any efficiency calculation will be wrong.
Cold-fermenter or cold-cellar samples read slightly high. A lager pulled at 4 °C against a 20 °C calibration reads about 0.0015 SG higher than the true value — small, but enough to misjudge attenuation by a couple of percent.
Summer brew days with ambient temperatures above 25 °C affect even room-temperature samples. If you're measuring in a warm garage or shed, knowing the actual sample temperature (rather than assuming "room temp") matters.
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.
Why do I need a hydrometer temperature correction?
A hydrometer is a density meter calibrated for a single temperature — usually 20 °C / 68 °F. The liquid you're measuring expands and contracts with temperature, which changes its density, which changes what the hydrometer reads. The correction factor tells you what your sample's specific gravity would have been if you had measured it at the calibration temperature, so the reading is comparable to a brewing chart, a previous batch, or another brewer's notes.
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 specific gravity temperature correction?
Small near room temperature, large near boiling. A 10 °C difference from calibration produces about a 0.001 SG change. A reading taken at 30 °C above calibration (mash sample, summer brew day) can need a +0.005 SG correction or more. Below the calibration temperature the correction is negative — a cold cellar sample reads slightly high.
Do I need to correct a reading taken at room temperature?
If your sample is within 2–3 °C of the hydrometer calibration temperature, the correction is under 0.0005 SG and you can usually ignore it. Outside that window — and certainly during brew day, fermentation crash-cooling, or anywhere the sample is warm or cold — the correction matters.
What is the correction factor formula?
This calculator uses the Lyons (1992) polynomial: factor(T_F) = 1.00130346 − 0.000134722 × T_F + 0.00000204 × T_F² − 0.00000000233 × T_F³. The corrected specific gravity is measured SG × factor(sample_T) ÷ factor(calibration_T). The same fit is used by Brewer's Friend, BeerSmith, and Brewfather and is accurate to within 0.0001 SG across normal brewing temperatures.
Does this work for both °C and °F?
Yes — enter the sample temperature in whichever unit your thermometer shows. The toggle above the inputs switches between °C and °F. Internally the polynomial uses Fahrenheit; the conversion is handled for you.
Should I correct a refractometer reading the same way?
No. Refractometers measure refractive index, not density. Most modern brewing refractometers have automatic temperature compensation (ATC) built into the prism, and the correction for any residual temperature offset is different from the hydrometer correction. Refractometers also drift in the presence of alcohol — a separate calculator handles the wort vs. fermenting-sample 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 or when a quick pre-pitch reading would otherwise hold up the rest of the brew day.