This calculator will give you an accurate specific gravity reading, correcting for the temperature of your wort / wine sample and the calibration temperature of your hydrometer.
Hydrometers will all be calibrated to work at a particular temperature. The most common calibration temperature for a home brewers hydrometer is 20°C / 68°F. This means if you take a gravity reading of a wort or must that is at a different temperature than the calibration temperature the reading will be inaccurate.
This is most apparent when you take a gravity reading of wort that being heated up to the boil. This pre-boil wort is most likely going to be a lot hotter than the calibration temperature of the hydrometer. When you try to take a gravity reading the hydrometer will sink a lot further into the wort giving you an inaccurate reading.
You will usually be able to find the calibration temperature of your hydrometer on the stem of the instrument. You can calibrate the hydrometer yourself by measuring the gravity of water at different temperatures. The specific gravity will only read 1.000 at the calibration temperature of your hydrometer.