Hydrometer Correction Calculator
Hydrometers only read true at their calibration temperature. Warm samples read low, cold samples read high. This corrects for it.
Hydrometers are calibrated at a specific temperature (usually 15°C or 20°C). If your wort sample is warmer or cooler than that, the reading will be off. The calculator above corrects for the difference — enter your reading, sample temp, and calibration temp, and it gives you the true gravity.
How We Calculate It
We use the Kell (1975) water density equation, a polynomial that models how water density changes with temperature. The correction compares the density of water at your sample temperature to the density at calibration temperature, then adjusts your gravity reading accordingly.
When does it matter?
At 30°C (86°F), a reading of 1.050 is actually closer to 1.052. At 40°C it's off by about 4 points. For OG readings taken right after chilling, the error is usually small. But if you're reading hot wort or very cold fermented beer, the correction matters for accurate ABV calculations.
Sources
- Kell, G.S. “Density, Thermal Expansivity, and Compressibility of Liquid Water.” J. Chem. Eng. Data 20(1), 1975.
Why does temperature change a hydrometer reading?
Hydrometers measure density, and water expands as it warms. A sample hotter than the calibration temperature is less dense, so the hydrometer floats lower and reads low; a colder sample reads high.
How much does temperature affect gravity?
Near the calibration point the effect is small, but it grows with the gap: at 30°C a 1.050 reading is really about 1.052, and at 40°C it's off by roughly 4 points. Hot wort and very cold beer need correction.
What calibration temperature should I use?
Use whatever is printed on your hydrometer — commonly 20°C (68°F) or 15.5°C (60°F). Choosing the wrong calibration temperature introduces a constant offset.
Do refractometers need the same correction?
No. Refractometers have their own temperature behavior and need an alcohol/wort correction after fermentation. This calculator is for hydrometers.