Since hydrometers are calibrated to be accurate at very specific temperatures, it is useful to adjust your hydrometer reading based on the difference between the temperature of your wort and your hydrometers calibrated temperature.
About The Calculator
This formula has been adjusted to accommodate hydrometers with different calibration temperatures. Generally speaking, older hydrometers will be calibrated at 59°F, while newer hydrometers are mostly calibrated to 68°F.
If you are unsure of what temperature your hydrometer is calibrated at, you can generally find this information on the hydrometer itself, its documentation/packaging, or of course checking with the manufacturer.
Hydrometer Temperature Correction Formula
CG = corrected gravity
MG = measured gravity
TR = temperature at time of reading
TC = calibration temperature of hydrometer
CG = MG * ((1.00130346 – 0.000134722124 * TR + 0.00000204052596 * TR – 0.00000000232820948 * TR) / (1.00130346 – 0.000134722124 * TC + 0.00000204052596 * TC – 0.00000000232820948 * TC))