Why correct hydrometer readings for temperature?
A hydrometer is accurate only at the temperature it was calibrated for, usually 60°F (15.6°C) or 68°F (20°C). Warm liquid is less dense, so the hydrometer sinks further and reads low. Cold liquid reads high. On a 1.050 reading, a sample at 80°F instead of 60°F reads about 2.5 points too low. That's enough to throw your ABV off by around 0.3%.
The calculator scales your reading by the density of water at the sample temperature relative to the calibration temperature:
Corrected = Reading × ρ(calibration temp) ÷ ρ(sample temp)
using the standard polynomial approximation for the density of water that most brewing software uses.
How to take a good reading
- Pull a sample into a clean test jar, so you never put the hydrometer into the fermenter.
- Spin the hydrometer to knock off bubbles, then let it settle.
- Read the scale at the bottom of the curved surface (the meniscus), with your eye level with the liquid.
- Take the sample's temperature, then enter both numbers above.
Use the corrected value in the ABV calculator, wine ABV calculator or mead calculator. Measuring in Brix instead? Use the refractometer calculator.
Method and sources
- Formula: corrected SG = measured SG × f(sample temperature) / f(calibration temperature), where f is a cubic in °F that tracks how the density of water changes with temperature.
- Check: water is 0.99904 g/mL at 60 °F and 0.99674 g/mL at 80 °F. That ratio turns a 1.050 reading at 80 °F into 1.0524, and the formula gives the same.
- Limits: the correction models water, not sugary must, and ignores the glass of the hydrometer expanding. It works best when the sample is within about 20 °F of the calibration temperature. For hotter samples, cool them and measure again.
Sources
Frequently asked questions
Why does temperature affect a hydrometer reading?
Liquids expand as they warm and become less dense, so a warm sample reads lower than it really is and a cold sample reads higher.
What temperature is my hydrometer calibrated to?
Most homebrew hydrometers are calibrated to 60°F (15.6°C) or 68°F (20°C). The calibration temperature is usually printed on the paper scale inside the hydrometer.
Can I measure a hot sample?
The correction is reliable up to about 100°F (38°C). For hotter samples, cool the sample first, because the error grows and hot liquid can damage a glass hydrometer.