Fermentation Temperature Correction Calculator
Correct your hydrometer gravity readings for fermentation temperature.
Hydrometers are calibrated at 20 C and give inaccurate readings at other temperatures.
Why a hydrometer reads wrong at the wrong temperature
A hydrometer floats based on liquid density. Wort expands when warm and contracts when cold, so the same beer reads slightly lower SG at 30°C than at 20°C, and slightly higher at 10°C. Almost all homebrew hydrometers are calibrated at 20°C (68°F). Read at 30°C and your gravity is off by about 0.002 SG, small enough to ignore on a porter, big enough to mis-quote your starting gravity on a session bitter.
The correction formula
The ASBC (American Society of Brewing Chemists) publishes a third-order polynomial:
correction_factor(T) = 1.313454 − 0.132674T + 0.002058T² − 0.00000263T³
where T is temperature in Fahrenheit. The calculator uses:
corrected_SG = measured_SG × (correction(T_measured) ÷ correction(68))
For a mental sanity check, a linear approximation works for most homebrew temperatures:
correction ≈ +0.001 SG per 5.5°C (10°F) above 20°C calibration
So a 1.048 reading at 30°C is really about 1.050; a 1.048 reading at 5°C is really about 1.045.
Correction at common temperatures
| Reading temperature | Correction |
|---|---|
| 5°C (41°F) | −0.003 SG |
| 10°C | −0.002 SG |
| 15°C | −0.001 SG |
| 20°C (calibration) | 0 |
| 25°C | +0.001 SG |
| 30°C | +0.002 SG |
| 40°C | +0.005 SG |
| 50°C | +0.009 SG |
The linear rule breaks down above 40°C; the polynomial stays accurate to about 50°C, which covers most reasonable measurement scenarios.
Why this matters for ABV
Apparent ABV ≈ (OG − FG) × 131.25
A 0.002 SG error in OG carries through as roughly 0.26% ABV miscalculation. For most styles that is acceptable, but it can push a session beer over the UK’s 4.0% duty band or send your IPA from 6.5% to 7.0% on the label. Always correct readings before doing anything official or commercial with them.
Practical tip
Use a refractometer for hot wort and a hydrometer for fermented beer. Or wait for your sample to reach 20°C before reading. This calculator is for when waiting is not practical: sampling in summer, sanitised cylinders just rinsed with hot water, that sort of thing.