Calculator
Hydrometer temperature correction
My sample was warm — what is the real gravity?
A hydrometer measures density, and density changes with temperature, so a warm sample reads lower than the same liquid would read cold. Instruments are calibrated to one specific temperature — usually 20°C or 60°F — and a reading taken at any other is wrong until corrected.
The error is small but not negligible: a sample straight out of a fermenter at 30°C can read several points low, which is enough to make a finished beer look unfinished or to misjudge an original gravity and therefore the final strength. The correction is worth applying to any reading taken outside a few degrees of calibration temperature.
Result
- Hydrometer temperature correction1.051
- Correction applied (gravity points)2.6
Working
Corrected SG = reading × (density factor at sample temp ÷ density factor at calibration temp)
| Step | Expression | Value |
|---|---|---|
| Sample temperature in °F | 30 °C | 86 °F |
| Correction factor | ratio of density factors | 1.002480 |
| Corrected gravity | 1.048 × 1.00248 | 1.0506 |
What it means
A warm sample reads low because the liquid is less dense. The correction is usually a point or two of gravity — enough to matter when deciding whether fermentation has finished.
Worked example
The example above starts from these values. It is rendered on the server, so the formula, assumptions and limitations are all present whether or not you change anything.
- Result1.051
- Correction applied (gravity points)2.6
Inputs
| Reading | 1.048 |
|---|---|
| Sample temperature | 30°C |
| Calibration temperature | 20°C |
How it is calculated
Corrected SG = reading × (density factor at sample temp ÷ density factor at calibration temp)
What each term means
| reading | The value shown on the hydrometer |
|---|---|
| density factor | A cubic polynomial in temperature, expressed in °F |
Step by step
| Step | Expression | Value |
|---|---|---|
| Sample temperature in °F | 30 °C | 86 °F |
| Correction factor | ratio of density factors | 1.002480 |
| Corrected gravity | 1.048 × 1.00248 | 1.0506 |
What this assumes
- The hydrometer is calibrated at the stated temperature — most are 20 °C, but 15.6 °C is also common.
Where it stops being reliable
- The correction is small at modest temperature differences and grows quickly beyond about 15 °C from calibration.
- It corrects for thermal expansion only, and assumes the hydrometer itself is accurate.
- The polynomial is a curve fitted to the thermal expansion of water. Wort and beer are not water, and dissolved extract changes how a sample expands, so the correction is an approximation that degrades as gravity rises. BeerHQ has not traced the fit to a primary source and does not cite one for it.
What the number means
A warm sample reads low because the liquid is less dense. The correction is usually a point or two of gravity — enough to matter when deciding whether fermentation has finished.
Physical data. Built on measured physical properties — the density of water at temperature, for instance. Accurate to the precision of the underlying tables.
Sources: The physical basis and its magnitude: NIST’s ITS-90 water density formulation (J Res NIST 97(3):335, 1992) covers 5–40 °C and puts the sensitivity at about 210 ppm per °C at 20 °C.
Not traced to a source: The cubic polynomial in °F applied here, which is a separate empirical fit.