How Do You Calculate Flow Capacities for Pipes?

Use the Hazen-Williams formula calculator found on the Calctool website to calculate flow capacities for pipes. Enter the pipe's diameter, length and drop. Use the drop down menus by each category to indicate the preferred unit of measurement for length. Click the calculate button to produce the velocity, discharge rate and pipe slope. To change the unit of measurement for speed when calculating velocity and discharge rate, use the drop down menus by each item.

If the pipe length and drop are unknown, but the slope is available, type a 1 in the calculator's space reserved for length. Enter the slope, which is the pipe's drop divided by its length, into the drop space. If the pipe is not circular and a physical diameter cannot be determined, but the hydraulic radius is identified, enter Rhx4 in the diameter space.

Roughness coefficient values indicate speed lost due to friction. This value represents an industry standard and is determined by the type of material from which the pipe was made. Concrete or cast iron pipes have a roughness coefficient of 100. Copper and plastic have roughness coefficients of 150. Steel and cement have coefficients of 120 and 140, respectively.

Calctool's calculator is meant to calculate the flow capacity for full pipes. The water is assumed to be at an ambient temperature and flowing freely.