Why does temperature affect hydrometer readings?

Water density changes with temperature — warm water is less dense than cold water. Since a hydrometer measures density, a sample that is warmer than the calibration temperature will give a reading that is too low. Cooler samples will read too high.

What temperature is my hydrometer calibrated for?

Most brewing hydrometers are calibrated at 20°C (68°F). Some older or laboratory hydrometers use 15°C (59°F) or 15.6°C (60°F). Check the markings on your hydrometer — it should indicate the calibration temperature.

How much difference does temperature make?

A sample at 30°C read on a hydrometer calibrated at 20°C will read about 0.002 SG too low. At 40°C, the error is about 0.005 SG. For pre-boil samples that may be very hot, always cool to near calibration temperature for the most accurate readings.

Should I cool my sample instead?

Cooling your sample to 20°C is the most accurate approach. However, if you need a quick reading during the brew day, this calculator lets you correct for the temperature difference without waiting.

Does this work for refractometer readings too?

No, this calculator is specifically for hydrometers. Refractometers have their own temperature compensation (ATC) built in. If your refractometer has ATC, it automatically adjusts for temperature. For refractometer readings during fermentation, use our Refractometer Correction Calculator instead.

BrewCalcs

Hydrometer Temperature Correction Calculator

Adjust hydrometer readings for sample temperature differences from calibration temperature.

Measured Gravity

Reading from your hydrometer

Sample Temperature

Hydrometer Calibration Temperature

Usually 20°C — check your hydrometer

How We Calculate This

Temperature Correction

The correction uses the density of water at different temperatures. The formula calculates the ratio of water density at the sample temperature versus the calibration temperature, and applies this ratio to the measured reading.

Best Practices

For the most accurate readings: cool your sample to within a few degrees of calibration temperature, ensure no air bubbles on the hydrometer, read at the bottom of the meniscus, and spin the hydrometer to dislodge bubbles before reading.

Frequently Asked Questions

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Last updated: 2025-02-20

All calculations are estimates. Always verify values and adjust based on your equipment and ingredients.