Tool
My cider was not at 20 °C — is my reading wrong?
Corrects a hydrometer reading taken away from the instrument’s calibration temperature. A warm sample reads low and a cold one reads high, and at fifteen degrees off the error is bigger than the change most readers are trying to detect.
Hydrometer temperature correction
The liquid, not the room. An active ferment runs warmer than the air around it.
Printed on the stem or the paper scale inside it. Almost always 20 °C; older instruments are sometimes 15 °C.
Corrected gravity
1.049
Correction applied: -1 points
How this is worked out
- Reading as taken: 1.050 at 12.0 °C
- Hydrometer calibrated at: 20.0 °C
- Correction: -1 points (-0.0013 SG)
- Corrected reading: 1.049
What this assumes (3)
- This corrects for water density only. It does not correct for anything else that was wrong with the reading — a sample that had not settled, a hydrometer with a dirty stem, or a reading taken off the meniscus rather than the surface.
- Dissolved carbon dioxide makes a fermenting sample buoyant and the reading low, and no temperature correction touches that. Degas the sample first if it is still working.
- Below about three degrees off calibration the correction is smaller than the error in reading the stem by eye, and applying it adds precision rather than accuracy.
Why a reading taken warm is wrong
A hydrometer measures density against water, and water’s own density changes with temperature. The instrument is calibrated at one temperature — printed on the stem, almost always 20 °C — and a reading taken anywhere else is comparing the sample against the wrong reference. A warm sample is less dense than it would be cold, so the hydrometer sits lower and the reading is low; a cold sample reads high.
At three or four degrees off calibration the correction is smaller than the error in reading the stem by eye and applying it adds precision rather than accuracy. At fifteen degrees off it is around five points, which is larger than the change most people are trying to detect when they take two readings days apart to find out whether a fermentation has stopped.
The correction handles water density and nothing else. It does not fix a sample that had not settled, a stem with a film on it, or a reading taken off the bottom of the meniscus rather than the surface — and it does nothing at all about dissolved carbon dioxide, which makes a fermenting sample buoyant and the reading low by considerably more than temperature ever does. The measuring guide covers both traps together.
Related tools
Where this reading comes from
Guides that reach this calculator, and what the number decides once you have it.