Water Pressure Loss Calculator

It uses the Hazen-Williams equation for water in full pipes. Loss per 100 ft in psi equals 4.52 times flow in gpm raised to the 1.852 power, divided by the C factor raised to the 1.852 power times the inner diameter in inches raised to the 4.8655 power. Total loss is that figure scaled by length over 100.

The menu maps to a Hazen-Williams C factor: smooth PEX, copper and PVC sit at 150, newer steel at 120, and old corroded steel or iron at 100. A lower C means a rougher inner wall and more friction, so choosing the wrong material can noticeably change the answer.

Enter the actual inner diameter, which is the water-carrying bore. Nominal sizes like half-inch or three-quarter-inch rarely match the true inside measurement, and because loss falls with diameter to nearly the fifth power, even a small bore error swings the result a lot.

No. It reports straight-run pipe friction only. For a vertical rise add roughly 0.433 psi per foot of height, and account for elbows, tees and valves by converting each to an equivalent length of pipe and adding it to your run.

Because the diameter term carries an exponent near 4.87, friction drops steeply as the bore grows. Moving from a 0.5 in to a 0.75 in inner diameter at the same flow can cut friction loss to a small fraction of the smaller pipe, which is often cheaper than boosting supply pressure.

It is a reliable engineering estimate for cool water in the typical flow range of home and irrigation systems. It is less exact for very hot water, very high velocities, or fluids other than water, where the Darcy-Weisbach method is preferred. Treat results as close estimates, not guarantees.

A common rule of thumb keeps velocity under about 8 feet per second to limit noise and erosion, and keeps total loss modest so fixtures still receive strong pressure. Use the loss per 100 ft figure to compare candidate pipe sizes on equal footing.

No. The calculation runs entirely in your browser using the same engine that powers the optional API, so nothing you type is uploaded or stored on a server.

The fields expect US gallons per minute, inches and feet. Convert metric figures first: 1 US gpm is about 3.785 liters per minute and 1 inch is 25.4 mm. Enter the converted values and the psi result stays valid.