Pipe Friction & Pressure Loss Calculator
Estimate how much pressure a run of pipe eats through friction. Enter the loss per 100 ft (a labeled figure that depends on pipe material, size and flow) and the developed length, and this tool scales it to your run.
Calculator
At 5.00 psi per 100 ft over a developed length of 80 ft, friction costs about 4.00 psi. The per-100-ft figure comes from the pipe material, size and flow (a labeled planning value) — confirm with a licensed plumber.
Friction is the quiet half of a pressure problem. Elevation loss is easy to picture, but a long, skinny, fitting-heavy run can quietly swallow just as much pressure — and unlike elevation, it gets worse the harder the water flows. Sizing this term is how a plumber decides whether a far bathroom needs a bump up in pipe diameter.
Formula
Friction loss scales linearly with length once you know the loss rate:
loss_psi = (per_100ft × developed_ft) ÷ 100
The per-100-ft figure comes from a friction chart (or a Hazen–Williams calculation) for the specific pipe material, inside diameter and flow rate — smaller pipe and higher GPM both raise it steeply. Developed length is the straight pipe plus an equivalent-length allowance for every elbow, tee and valve, which behave like extra feet of pipe.
Worked example
A run loses about 5 psi per 100 ft at the design flow, and the developed length works out to 80 ft.
- loss = (5 × 80) ÷ 100 = 4.0 psi.
Add that 4 psi to the elevation loss in the pressure tool to see the pressure genuinely available at the fixture. If the number is uncomfortably high, the fix is usually larger pipe (which drops the per-100-ft rate sharply) or a shorter, straighter route.
What drives friction loss
Two knobs drive friction loss: the rate (psi per 100 ft) and the length. The rate rises fast as pipe gets smaller or flow gets higher — halving the diameter can multiply the loss several times over — which is why the cheapest cure for a weak far fixture is often one size larger pipe rather than a pump.
Because fittings add resistance, plumbers use developed length: the measured straight pipe plus an equivalent length for each elbow, tee and valve (a 3/4 in elbow behaves like roughly two extra feet of pipe, for example). A tidy way to keep pressure up is to minimize direction changes and avoid running long branches in small pipe. The per-100-ft value you enter here is a labeled planning figure — pull the exact number from a friction chart for your material and flow, and confirm the final design with a licensed plumber.
Reference table
| Developed length | Friction loss |
|---|---|
| 25 ft | 1.25 psi |
| 50 ft | 2.50 psi |
| 80 ft | 4.00 psi |
| 100 ft | 5.00 psi |
| 150 ft | 7.50 psi |
At your entered rate of 5.00 psi per 100 ft. Larger pipe lowers the rate; more fittings raise the developed length.