To calculate the water flow rate through a given pipe size, multiply the area of the inner cross-section of the pipe by the velocity of the water. The velocity of the water is required for this calculation.
Pipe sizes are usually given in diameters, as most of the pipes in use are round. To determine the area of the inner cross-section of the pipe, first divide the diameter by two in order to find the radius. The area of a circle is equal to pi x radius x radius. Multiplying the area by the velocity of the water yields the volume of water flow per unit of time.