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 0.003 SG, small enough to ignore on a porter, big enough to mis-quote your starting gravity on a session bitter.

The correction formula

The density ratio between the reading temperature and the calibration temperature is a third-order polynomial in Fahrenheit:

correction_factor(T) = 1.00130346 − 0.000134722T + 0.00000204053T² − 0.00000000232821T³

corrected_SG = measured_SG × (correction(T_measured) ÷ correction(68))

Those coefficients look oddly tiny, and that is the point. Water changes density by only about 1% across this whole range, so the factor never strays far from 1.0. If you ever see a hydrometer correction formula whose factor comes out at 3, or negative, the coefficients have been mangled somewhere.

For a mental sanity check, a rough linear approximation works over the usual brewing range:

correction ≈ +0.001 SG for every 4°C above the 20°C calibration point

It is not truly linear. The gap widens as it gets hotter, from about 0.00023 SG per degree at 25°C to 0.00031 at 40°C, so the rule of thumb understates a hot sample.

So a 1.048 reading at 30°C is really about 1.051; the same reading at 5°C is really about 1.046.

Correction at common temperatures

Figures below are for a reading near 1.048. The correction scales with the gravity itself, so a 1.090 wort shifts almost twice as far as a 1.010 finished beer at the same temperature.

Below about 5°C the curve flattens out and then reverses, because water is at its densest near 4°C. The polynomial holds to roughly 50°C, which covers any sample you can reasonably float a hydrometer in.

Why this matters for ABV

Apparent ABV ≈ (OG − FG) × 131.25

A 0.003 SG error in OG carries through as roughly 0.39% 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.