Hydrometer Temperature Correction Calculator
Enter your hydrometer reading and the sample temperature to get the corrected specific gravity at the calibration temperature. Brix and Plato equivalents included. Nothing uploaded.
Reading
Corrected Reading
Learn more: correcting a hydrometer reading for temperature
Hydrometers are calibrated at one temperature
A hydrometer infers density from how far it sinks, and it is built to read correctly at one calibration temperature. Brewfather's hydrometer correction guide says that is typically 15.6 degrees C (60 F) or 20 degrees C (68 F). Check yours, since it is usually printed on the instrument. The calculator defaults to 20 degrees C.
A warm sample reads low
A warm liquid expands and becomes less dense, so the hydrometer sinks further and reads lower than the true gravity. Brewfather says warmer samples read lower than the actual gravity. The correction therefore adds points. A cold sample reads high and gets points taken off.
The calculator uses the standard homebrewing formula: corrected gravity = measured gravity x f(sample temperature) / f(calibration temperature), where f is a cubic fit for the density of water with temperature in Fahrenheit. A 1.048 reading at 30 degrees C on a 20 degree C hydrometer corrects to 1.0506. A 1.050 reading at 32 degrees C (90 F) corrects to 1.0532, and a 1.020 reading at 5 degrees C corrects down to 1.0182.
That matters when you pull a sample straight from a warm fermenter. Left uncorrected, the reading is too low, so it looks as if fermentation has gone further than it has.
SG, Brix and Plato
The corrected gravity is also shown as degrees Brix and degrees Plato, two scales for the percentage of sugar by weight. They come from polynomial fits to the corrected specific gravity, so 1.0532 is about 13.2 Brix and 13.1 Plato.
FAQ
Do I need to correct every reading?
Only when the sample is not at your hydrometer's calibration temperature. Letting the sample cool to that temperature before reading avoids the correction, though that is not always practical during fermentation.
Why is the correction not a flat amount per degree?
The density of water does not change in a straight line with temperature. The formula uses a cubic fit, so the correction grows faster per degree as the sample gets warmer.
What is the difference between SG, Brix and Plato?
Specific gravity compares the liquid's density with water. Brix and Plato both describe the sugar content as a percentage by weight. The calculator derives them from the corrected gravity.