See below for the formula that the brewing industry uses.
All hydrometers are calibrated to be accurate at a given temperature.
Most new hydrometers are calibrated for 68 °F (20 °C).
The density of fluids changes as their temperature changes, so you will get an inaccurate reading if you don’t measure the specific gravity at your hydrometer's calibration temperature.
You must correct each of your readings for temperature differences before you can compare them.
The formula to correct a reading for any hydrometer calibration temperature is as follows:
You may find it easier to use this calculator from Brewer's Friend.