Friction Loss Calculator

Estimate how much pressure water loses flowing through a pipe, using the Hazen–Williams equation.

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PSI Lost

Head loss
Per 100 ft
Velocity
Elevation
Total loss
Inside diameter
Pressure Needed

Pipe Runs

Items you add are saved in this browser.

Run	Pipe	GPM	Length	PSI Lost	Remove

How to Calculate Friction Loss

Water loses pressure rubbing against the pipe wall. The Hazen–Williams equation is the standard way to estimate it for water at ordinary temperatures. The C factor describes how smooth the pipe is.

Head loss (ft per 100 ft) = 0.2083 × (100 ÷ C)^1.852 × gpm^1.852 ÷ ID^4.8655

PSI loss = feet of head × 0.433

Loss grows almost with the square of the flow and falls steeply with pipe size: going up one pipe size often cuts friction loss by more than half.

Worked Example

Typical C Factors

Pipe	C
PVC, CPVC, PEX, polyethylene	150
Copper	130–140
New steel	120
Cast iron, new	130
Steel or cast iron, old or corroded	80–100

Loss per 100 Ft. in Schedule 40 at C = 150

GPM	¾″	1″	1½″	2″

PSI per 100 ft, using nominal inside diameters. Published irrigation charts use average-wall IDs, which are slightly smaller, so they read a few percent higher (for example 2.63 psi vs. 2.40 psi for 1″ at 10 gpm). Enter the chart ID under Other to match one exactly.

Tips

Hazen–Williams is meant for water between about 40 °F and 75 °F. For hot water, other liquids or very small tubing, a Darcy–Weisbach calculation is more accurate. Add equivalent lengths for fittings from the fitting maker's data or a friction loss handbook.