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.
Why temperature matters
A hydrometer floats higher in cold liquid and lower in warm liquid, because water becomes less dense as it warms. If your hydrometer is calibrated at 20 °C and your sample is at 30 °C, the reading on the scale is lower than the true gravity — typically by about 2 points (0.002 SG).
At mash or boil temperatures the error is much larger. A wort sample at 65 °C can read 8–10 points low, which would make your brewhouse efficiency look worse than it is.
The correction formula
The standard correction uses a polynomial that models water’s density as a function of temperature:
SGcorrected = reading × poly(Tsample) / poly(Tcal)
where poly(T) = 1.00130346 − 0.000134722124 × T + 0.00000204052596 × T² − 0.00000000232820948 × T³, and T is in °C.
Quick reference
At a calibration temperature of 20 °C:
- 20 °C — no correction needed
- 25 °C — add about 1 point
- 30 °C — add about 2 points
- 40 °C — add about 5 points
- 60 °C — add about 9 points
These are approximate — the actual correction depends on the gravity of the sample, not just the temperature.
When to use a refractometer instead
A refractometer gives a reading in seconds from a single drop and is largely unaffected by temperature (most have automatic temperature compensation). For pre-fermentation readings it is faster and more convenient than a hydrometer. After fermentation starts, alcohol shifts the refractometer reading and you need the refractometer correction calculator to get a true gravity.
Hydrometer Temperature Correction — questions
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.
Gravity readings that correct themselves
Log a gravity reading in Brewgravity with the sample temperature and the correction is applied automatically — one fewer step between kettle and notebook.
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