How much pressure does your pipe run lose to friction?

Common questions

How do you calculate friction loss in a pipe?

The standard method for water is the Hazen-Williams equation. In SI: head loss h (metres) = 10.67 × L × Q^1.852 ÷ (C^1.852 × d^4.87), where L is length (m), Q is flow (m³/s), d is the inside diameter (m) and C is the pipe's roughness coefficient. Enter the flow, inside diameter, run length and material above and the calculator does it for you, in feet or metres of head plus the equivalent pressure drop.

What is a Hazen-Williams C value?

C is the Hazen-Williams roughness coefficient — a standard published number for each pipe material that says how smooth it is. Higher C means a smoother pipe and less friction: plastic (PVC, PEX) is about 150, copper about 140, new steel or cast iron about 120–130, and old rough iron drops toward 100 as it corrodes. It is a physical property of the pipe, not a code value.

How much pressure do you lose per 100 feet of pipe?

It depends entirely on the flow, the bore and the material — that is what the calculator works out. As a feel for it, 10 GPM through a 1-inch copper line loses roughly 3–4 psi (about 8 feet of head) per 100 feet. Halve the pipe size and the loss climbs steeply, because friction loss rises sharply as the diameter shrinks.

Does a smaller pipe cause more friction loss?

Yes — dramatically. Friction loss rises with roughly the inverse fifth power of the diameter, so a small drop in bore causes a large jump in loss. Going up one pipe size is by far the most effective way to cut friction loss and recover pressure on a long run.

How do you account for fittings and valves?

Elbows, tees and valves each add loss, usually handled by the equivalent-length method: look up the equivalent straight-pipe length for each fitting, add them to the real pipe length, and put that total length into the calculator. This tool computes the straight-run friction loss; add the fittings' equivalent length to the length input.