Irrigation in the Pacific Northwest

Pipe Water Velocity and Minimum Pipe Diameter

Used to calculate the velocity of water in a pipe. Bigger pipe is more expensive, but keeping the water velocity low is important to limit pressure losses due to friction, water hammer, and pipe movement due to water momentum changes inside the pipe. Use the second form to calculate the inside diameter of a pipe at a water velocity of 5 ft/sec. 5 ft/sec is a somewhat arbitrary velocity threshold that some designers use because beyond that the friction losses, danger of water hammer, and pipe movement due to water momentum changes are deemed to be too high. This 5 ft/sec threshold is a simple method of selecting pipe size, but a better method is to use allowable pressure losses due to friction and elevation change and to include protections against water hammer and pipe movement in the trenches (blocking). For example larger friction losses in a pipe going downhill may be allowable or even desirable. Learn more about the units used on this page.

Water Velocity

Flow Rate:
Pipe Inside Diameter:
Water Velocity:
ft/sec (fps)


Minimum Pipe Diameter to Limit Velocity to 5 ft/sec

Flow Rate:
Minimum Pipe Inside Diameter (pick the next largest pipe size):



The Equation

This calculator uses this formula to determine the water velocity inside of a pipe.


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