# Hydrometer Temperature Correction

A hydrometer measures density by displacement, but the density of water changes with temperature, so a reading taken at a temperature other than the calibration point needs correcting. The correction uses an ASHA polynomial that models water's thermal expansion: SG_corrected = reading × poly(T_sample) / poly(T_cal). A sample at 30 °C reads roughly 2 points low on a hydrometer calibrated at 20 °C.

Hot wort reads lower than its real gravity. This tells you how much lower, so you can take a reading without waiting for it to cool.

HTML version: https://brewgravity.com/calculators/hydrometer-correction/

## Inputs

- **Hydrometer reading (SG)** (min 0.98, step 0.001) — default: 1.050
- **Sample temperature (°C)** (step 0.5) — default: 30
- **Calibration temperature (°C)** (step 0.5) — default: 20

## Outputs

- **corrected**: Corrected SG
- **plato**: Corrected °Plato
- **delta**: Correction

## FAQ

### Does the correction matter for a normal gravity reading?

At room temperature, barely — a degree or two above 20 °C shifts the reading by less than a point. But a reading taken from hot wort straight out of the mash or boil kettle can be off by 5–10 points, which is enough to change your efficiency figure significantly.

### What temperature is my hydrometer calibrated at?

Most brewing hydrometers are calibrated at 20 °C (68 °F). Some older or scientific models use 15.56 °C (60 °F). It is usually printed on the scale or in the documentation.

### Should I cool my sample before reading it?

If you can, yes — a cooled sample gives a direct reading with no correction needed. But if you want a quick check during the mash or boil, this correction lets you trust a hot reading.

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Source: https://brewgravity.com — Free hydrometer temperature correction calculator using the ASHA polynomial to correct a specific gravity reading for sample temperature against the hydrometer's calibration temperature.