Gravity Temperature Correction

A hydrometer is set to read correctly at one specific temperature, usually printed on the instrument or its paperwork. Liquid expands as it warms, so a warm sample is less dense and the hydrometer floats lower in it, giving a reading below the true value. A cold sample contracts and does the reverse, floating higher and reading artificially high. The correction rescales the reading using the known density of water at both the sample temperature and the calibration temperature.

The effect is small near room temperature but grows quickly the further the sample sits from calibration temperature, which matters most right after a hot boil or mash, when a sample can easily be 30-40 degrees away from where the hydrometer was designed to read. For gravity readings that feed an ABV calculation, correcting both the original and final reading keeps the alcohol figure honest, since an uncorrected error in either reading shows up directly as an error in the calculated ABV.

60F has long been the standard calibration point for many brewing hydrometers, largely a holdover from historical brewing industry convention. Wine and laboratory instruments more often use 68F, which is exactly 20C, following the metric convention more common in winemaking and scientific measurement. Neither is more correct than the other; they're simply two different reference points a hydrometer can be built around.

Because of this, the same physical hydrometer reading needs a different correction depending on which calibration temperature it was actually built for, which is why checking your specific instrument rather than assuming a default matters if you want an accurate corrected figure.