Pipe Friction Loss Calculator
Calculate pressure loss from pipe friction using the Hazen-Williams formula, from flow rate, pipe material (C-factor), diameter, and length.
How this calculator works
Water loses pressure as it pushes through a pipe, and the Hazen-Williams formula is the standard way to estimate how much. It combines four things: how much water is flowing, how rough or smooth the pipe’s interior is, how wide the pipe is inside, and how long the run is.
The calculator first finds the friction loss per 100 feet of pipe based on flow, C-factor, and diameter. Then it scales that figure to the actual length of pipe you’re checking, giving you a total pressure loss for the run.
Formula: Hazen-Williams friction loss per 100 ft (psi) = 4.52 × flow(GPM)^1.852 ÷ (C-factor^1.852 × diameter(in)^4.87). Total loss = that figure × (pipe length ÷ 100).
Worked example
- Flow: 50 GPM
- C-factor: 140 (PVC/copper)
- Inside diameter: 2 inches
- Pipe length: 200 ft
- Loss per 100 ft ≈ 0.023 psi
- Total loss ≈ 0.046 psi
Notes
The C-factor represents pipe roughness — smooth materials like PVC and copper use about 140-150, while older or rougher pipe (galvanized steel, cast iron) uses a lower C-factor and loses more pressure for the same flow. Friction loss grows fast with flow rate and shrinks fast with a larger diameter, so small changes in either input can move the result more than you’d expect.
How to use
Enter the flow rate you expect through the pipe, the C-factor for your pipe material, the inside diameter, and the total length of the run. The calculator returns the friction loss per 100 feet and the total loss over your pipe length, so you can check whether pressure at the far end will still be adequate.
The Hazen-Williams formula is a standard water-flow approximation for typical pressure ranges. For critical system design, confirm with a plumbing engineer.
Frequently asked questions
What is the Hazen-Williams formula used for?
It's a widely used formula for estimating how much water pressure is lost to friction as water moves through a pipe. It accounts for the flow rate, the pipe's roughness (its C-factor), the inside diameter, and the length of pipe. Plumbers and engineers use it to check whether a pipe run will deliver enough pressure at the far end of a system.
What C-factor should I use for my pipe material?
C-factor represents pipe roughness on a scale where higher numbers mean smoother pipe and less friction loss. Smooth materials like PVC and copper typically use a C-factor around 140 to 150. Older or rougher materials, such as galvanized steel or cast iron, use a lower C-factor and lose more pressure for the same flow. Check your pipe manufacturer's data for the exact value.
Why does friction loss increase so quickly with flow rate?
In the Hazen-Williams formula, flow rate is raised to the power of 1.852, not multiplied in a straight line. That means doubling the flow through a pipe more than triples the friction loss, rather than simply doubling it. This is why oversized fixtures or multiple fixtures running at once can cause a noticeable pressure drop even on a well-sized pipe.
How much does a larger pipe diameter reduce friction loss?
Diameter has an even bigger effect than flow rate because it's raised to the power of 4.87 in the formula. Increasing the inside diameter even modestly shrinks friction loss fast, which is why upsizing a pipe is often the simplest fix for a low-pressure complaint at the end of a long run.
Estimates only. Verify quantities with your supplier before purchasing.