Fermentation Temperature Correction Calculator

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

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.