Hydrometer temperature correction calculator
A hydrometer is calibrated for one temperature, usually 20 °C (68 °F) or 15.6 °C (60 °F). Read it in a warmer or colder sample and the number is off, because liquids expand as they warm. Enter the reading, the sample temperature and the calibration temperature printed on the hydrometer to get the corrected gravity.
Runs in your browser: nothing you enter is sent anywhere.
How to use
- Read the hydrometer at the bottom of the meniscus and type the gravity, for example 1.050.
- Choose °C or °F and enter the temperature of the sample at the moment you read it.
- Enter the calibration temperature printed on the hydrometer.
- Read the corrected gravity and how many points the correction added or removed.
How it works
The usual hydrometer correction multiplies the reading by the ratio of water density at the sample and calibration temperatures, from a cubic fit in °F:
corrected = reading × d(T) ÷ d(Tc), where d(T) = 1.00130346 − 0.000134722124 T + 0.00000204052596 T² − 0.00000000232820948 T³.
Degrees Plato come from the corrected gravity: °P = −616.868 + 1111.14 SG − 630.272 SG² + 135.997 SG³.
Example
A sample reads 1.050 at 86 °F on a hydrometer calibrated at 68 °F (30 °C and 20 °C):
- d(86) = 1.0033283 and d(68) = 1.0008458;
- corrected = 1.050 × 1.0033283 ÷ 1.0008458 = 1.0526, shown as 1.053;
- that is +2.6 points, and about 13 °P.
Tips
- Spin the hydrometer gently to free bubbles before reading, and read at eye level.
- Check the hydrometer in plain water at its calibration temperature: it should read 1.000; if not, add or subtract the difference from every reading.
- Hot wort can crack a glass hydrometer and test jar: cool the sample first when you can.
Frequently asked questions
Why does temperature change a hydrometer reading?
Warm wort is less dense than the same wort at the calibration temperature, so the hydrometer sinks deeper and reads low. A cold sample reads high. The correction scales the reading by the density of water at the two temperatures.
How big is the correction?
Small near the calibration temperature and growing as the gap widens. A 1.050 reading at 30 °C (86 °F) on a 20 °C (68 °F) hydrometer is really about 1.053, 2.6 gravity points more. Above about 50 °C the formula still runs, but cooling the sample before measuring is more reliable.
What is a gravity point?
Brewers count thousandths of specific gravity as points: 1.050 is 50 points, and going from 1.050 to 1.053 is a change of 3 points. The calculator shows the change to a tenth of a point.
Does this work for a refractometer?
No. Most refractometers compensate for temperature on their own, and after fermentation starts their readings need a different correction for alcohol. This calculator is for floating glass hydrometers.