Gravity temperature correction adjusts a hydrometer reading taken at one temperature to what it would read at the instrument's actual calibration reference temperature — commonly 60°F (15.6°C) or 68°F (20°C) depending on the specific hydrometer. Liquid expands as it warms, which changes its density and throws off a reading taken away from that reference point, even though nothing about the actual sugar content changed.
Why a hot sample reads misleadingly low
A hydrometer sample pulled straight from a hot kettle or an actively-conditioning fermenter and read without correction shows an artificially low gravity, because the warm, expanded liquid is genuinely less dense at that moment than it would be once cooled — a brewer who doesn't correct for this can misjudge OG as lower than it really is, or misjudge fermentation as further along than it actually is.
A small correction that adds up across a brew day
The gap is modest at any single reading but genuinely matters when comparing readings taken at different temperatures against each other — an OG read hot and an FG read cold, both uncorrected, can produce an apparent attenuation figure that's meaningfully wrong even though each individual reading felt close enough at the time. Cooling samples to a consistent temperature before reading, or applying a correction table, avoids the error entirely.