This calculator requires the use of Javascript enabled and capable browsers. The formula used in the calculation process is [(Head(Ft.) X S.G.)/2.31]. Enter the values for head (in feet) and specific gravity of the fluid being pumped to yield pounds per square inch. Specific gravity is the heaviness of a substance compared to that of water, and it is expressed without units. In the metric system specific gravity is the same as in the English system. If something is 7.85 times as heavy as an equal volume of water (such as iron is) its specific gravity is 7.85. Its density is 7.85 grams per cubic centimeter, or 7.85 kilograms per liter, or 7.85 metric tons per cubic meter. By definition, the specific gravity of water is 1. To calculate the head, use a specific head calculator, such as ours.