Select which parameter you want to compute and fill in the known values of the two other parameters - click "Calculate!" to get the answer. Be sure to set the hydrometer calibration temperature.
Be sure to click the option button for the field you want to solve for. If you hwant to know how much more extract you would need to increse the gravity a specified amount just run the calculator for the desired gravity and note the amount of extract needed. Then run it again again for the current gravity and subtract the amount from the first amount. The resulting number will give you the amount of extract needed to raise the gravity. (Measuring specific gravity and volume requires a hydrometer and thermometer, or refractometer, and a calibrated kettle or other brewing vessel.)