DemijohnHydrometer temperature correction
A hydrometer reads true only at the temperature it was calibrated for. Enter the reading and the sample temperature to get the corrected gravity.
Result
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How it’s worked out
Why a correction at all
Warm liquid expands and gets lighter, so the hydrometer sinks deeper and reads low. Cold liquid reads high. The glass of the hydrometer expands a little too.
Why not the usual formula
Most calculators use a formula fitted to the expansion of pure water. Sugar solutions expand more than water, so at must gravities that formula under-corrects by 0.2 to 0.5 points for a sample 5 to 10 degrees off. Demijohn uses NBS Table 110, measured for sugar solutions from 0 to 35 % sugar and 10 to 35 °C, with the glass included. Between rows the table is interpolated.
For a 60 °F or other hydrometer the table correction at its calibration temperature is subtracted, the transfer rule printed under the NBS table.
Outside 10 to 35 °C
From 0 to 10 °C and 35 to 40 °C there is no table, so a water-density ratio is used and the result is marked as less certain. Beyond 0 to 40 °C the calculator stops: bring the sample to 15 to 25 °C and read again.
Finished wine
Alcohol expands more than water too. At 5 degrees off, the correction for a finished dry wine is about 0.3 points short. That is below what most hobby hydrometers can show, but read finished wine near the calibration temperature when you can.
Sources
- Bates et al., NBS Circular C440 (1942), Tables 109, 110 and 114
- OIV-MA-AS2-01B, Tables V and VI: hydrometer temperature corrections
- Tanaka et al., Metrologia 38 (2001) 301: density of water
Doses are estimates. Add in steps, test again, follow the product label. For personal use only.
Checked against the sources: October 25, 2026