Collect the information you need for the calculation. This is the pressure drop (p), the inside diameter (D) of the pipe, the length of the pipe between the pressure measurements (L), and the roughness (k) of the inside of the pipe.
Convert the variables to feet. Multiply pressure in psi by 2.31 or pressure in millimeters of mercury (Hg) by 1.13 to get the pressure drop in feet of head. Divide the inside diameter or length of pipe by 12 if either one is in inches.
Divide the roughness (k) by the Inside diameter of the pipe (D). Note that the roughness (k) of wrought iron commercial steel pipe is 0.00015 feet. Convert the roughness to feet if it is given in inches. Locate this relative roughness (k/D) on the Moody resistance diagram (see Resources). Go horizontally across the diagram and read the friction coefficient (f).
Multiply the area of the inside of the pipe (pi/4 x D squared) times the square root of ((64.4 x D x p)/(f x L)) to find the flow (Q) in the pipe in cubic feet per second. Note: This calculation assumes water flowing in a completely filled horizontal pipe at a "turbulent" velocity.