Water Pressure Loss Calculator
Estimate how much water pressure a pipe run loses to friction. Enter the flow in gpm, the inner pipe diameter, the length and the pipe smoothness to get the loss per 100 feet and the total in psi.
How to calculate water pressure loss
- Enter the flow rate through the pipe in gallons per minute.
- Enter the inner pipe diameter in inches and the run length in feet.
- Pick a C factor for the pipe material, then read the friction loss in psi.
Examples
10 gpm through 100 ft of half-inch PEX
10 gpm, 0.75 in inner diameter, 100 ft, C factor 150
About 0.12 psi per 100 ft and 0.12 psi total
Frequently asked questions
What formula does this calculator use?
It uses Hazen-Williams. Loss per 100 ft in psi equals 4.52 times flow in gpm to the 1.852 power, divided by the C factor to the 1.852 power times the diameter in inches to the 4.8655 power.
What C factor should I use?
Smooth PEX, copper and PVC are about 150, newer steel about 120, and old corroded steel or iron about 100. A lower C factor means rougher pipe and more loss.
How does pipe size affect pressure loss?
Strongly. Loss falls with the diameter to nearly the fifth power, so going up one pipe size can cut friction loss to a fraction of the smaller pipe.
Does this include height or fitting losses?
No. This is straight-run friction only. Add about 0.433 psi per foot of rise and account for elbows and valves separately as equivalent length.
What is a reasonable pressure loss target?
Many designers keep velocity under about 8 ft per second and total loss modest so fixtures still get good pressure. Use the loss per 100 ft to compare pipe sizes.
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