How to use the hydrometer temperature correction calculator
- Enter the hydrometer reading as specific gravity, such as 1.052.
- Choose Fahrenheit or Celsius and enter the sample temperature when you took the reading.
- Enter the calibration temperature, which is printed on the hydrometer. Many are 60 F (15.6 C) and some are 68 F (20 C).
- Read the corrected specific gravity and the correction in gravity points.
Formula
f(T) = 1.00130346 - 0.000134722124 T + 0.00000204052596 T^2 - 0.00000000232820948 T^3
corrected = reading x f(sample temperature) / f(calibration temperature)
T is in degrees Fahrenheit
The polynomial follows how the density of water changes with temperature. Celsius temperatures are converted to Fahrenheit first. The result is shown to four decimals.
A warm sample read at 82 F
A reading of 1.052 taken at 82 F with a hydrometer calibrated at 60 F gives f(82) = 1.002693 and f(60) = 1.000063, so the corrected gravity is 1.052 x 1.002693 / 1.000063 = 1.0548. The correction is +2.8 points, because warm liquid is less dense and the hydrometer sinks lower than it would at its calibration temperature.
A reading of 1.050 at 100 F corrects to 1.0561, and 1.040 at 50 F with a 68 F hydrometer corrects to 1.0385, a drop of 1.5 points. Checked against the Kell formula for the density of water, the corrected values agree to within 0.0002.
Why temperature matters
A hydrometer floats in a liquid, and how high it floats depends on the liquid's density. Density changes with temperature, so the same wort or must reads differently when warm and when cool. The instrument is made to be accurate at one temperature, its calibration temperature, and readings taken at any other temperature need a correction.
The simplest fix is to cool the sample to the calibration temperature before you read it. When that is not practical, as with a hot sample from a boil or a fermenter that is warm, a correction gives a close estimate. For the best reliability, cool the sample if you can.
How good the correction is
The formula treats the liquid as water, whose density change with temperature is well known. Sugar solutions behave very nearly the same over the usual range, so the result is close but not exact, and the difference grows as the sample gets hotter. The page accepts temperatures from 32 to 176 F, which is 0 to 80 C, where the approximation is reasonable.
The corrected value is an estimate, and it is for information only. For related tasks, the ABV Calculator turns gravity readings into alcohol by volume, the Simple Syrup Calculator mixes a sugar solution of a known ratio, and the Brine Calculator works out salt for a given strength.
Assumptions and limits
- The reading is specific gravity between 0.99 and 1.2. A reading in points, such as 52, is not accepted.
- Temperatures are from 32 to 176 F or 0 to 80 C, with the same unit for the sample and the calibration.
- The correction treats the liquid like water. It is an estimate and not a laboratory result.
- Nothing is stored or uploaded.
Frequently asked questions
How do I correct a hydrometer reading for temperature?
Multiply the reading by the water factor at the sample temperature divided by the factor at the calibration temperature. A 1.052 reading at 82 F on a 60 F hydrometer corrects to 1.0548.
What is the calibration temperature of my hydrometer?
It is printed on the hydrometer or its tube, often 60 F (15.6 C) or 68 F (20 C). If it is missing, check the maker's documentation, since using the wrong value changes the correction.
Does a warmer sample read higher or lower?
Lower. Warm liquid is less dense, so the hydrometer sinks further and shows a lower number. The correction therefore adds points when the sample is warmer than the calibration temperature.
Is it better to cool the sample instead?
Yes, when you can. Reading at the calibration temperature needs no correction at all. The calculator is for the times when cooling is not practical.