Pipe Friction Loss Calculator
A pipe friction loss calculator applies Hazen-Williams: 50 GPM through 200 ft of 2 in pipe at C 140 shows 2.30 psi per 100 ft and 4.59 psi in total.
Pipe friction loss calculator: the Hazen-Williams formula
Water pushing through a pipe rubs against the wall and loses pressure along the way. A pipe friction loss calculator estimates that loss with the Hazen-Williams formula, using the flow rate, the pipe’s smoothness (the C-factor), the inside diameter, and the pipe length.
The tool first computes the loss per foot, and then scales it to 100 ft and to your length.
Loss per foot (psi) = 4.52 × GPM^1.852 ÷ (C^1.852 × diameter^4.87). Loss per 100 ft = loss per foot × 100. Total loss = loss per foot × length.
What each input does
Flow rate is how much water moves through the pipe each minute. A higher flow pushes harder against the wall.
The C-factor is the smoothness. A higher C means a smoother pipe and less loss.
Diameter is the inside width of the pipe, not the nominal size on the label. A pipe with a thick wall has a smaller inside diameter than its name suggests.
Length is the straight run. A longer run has more wall to rub against.
Worked examples
Example 1, the default values. 50 GPM, C = 140, 2 in inside diameter, 200 ft.
- Loss per 100 ft: 2.30 psi
- Total: 2.30 × 200 ÷ 100 = about 4.59 psi
Example 2, a longer, bigger run. 100 GPM, C = 150, 3 in inside diameter, 300 ft.
- Loss per 100 ft: 1.01 psi
- Total: 1.01 × 300 ÷ 100 = about 3.04 psi
How the inputs scale
Each row below changes one input from example 1 and keeps the others the same.
| Change | Loss per 100 ft (psi) |
|---|---|
| Default: 50 GPM, C 140, 2 in | 2.30 |
| Flow 25 GPM | 0.64 |
| Flow 75 GPM | 4.86 |
| Flow 100 GPM | 8.29 |
| C 100 (rough pipe) | 4.28 |
| C 120 | 3.06 |
| C 150 | 2.02 |
Doubling the flow from 50 to 100 GPM raises the loss by a factor of 3.6. Moving from C 140 to C 100 nearly doubles it.
Diameter matters most
All rows use 50 GPM and C 140.
| Inside diameter | Loss per 100 ft (psi) |
|---|---|
| 1.5 in | 9.32 |
| 2 in | 2.30 |
| 2.5 in | 0.77 |
| 3 in | 0.32 |
Going from 1.5 in to 2 in cuts the loss to about a quarter. Because the exponent is 4.87, a small bump in diameter beats a big drop in flow.
Picking a C-factor
| Pipe | Typical C range |
|---|---|
| PVC and other plastic | 140 to 150 |
| Copper | 140 to 150 |
| Galvanized steel, newer | around 120 |
| Older steel or cast iron | 100 and lower |
Pipes lose smoothness with age, so a design with some margin is wiser than one that works only at the new-pipe value.
Measuring your pipe
Find the inside diameter from the pipe’s schedule or class, not from the label size. Measure the run along the pipe, including vertical sections, but leave out the fittings. You can approximate fittings by adding an equivalent length of pipe for each elbow or valve, using your supplier’s table.
What the tool leaves out
- Fittings, valves, filters, and meters
- Elevation gain or loss
- Pressure needed at the outlet
- Pump curves and available head
Common mistakes
- Using the nominal diameter in place of the inside diameter.
- Entering the flow in the wrong unit. The box wants gallons per minute.
- Using a high C for old pipe.
- Reading the per-100-ft number as the total.
Using the result
Subtract the total loss from the pressure at the start of the run. What remains is what reaches the far end before you account for fittings and any climb in elevation. If the left-over pressure is too low for the sprinkler head, fixture, or tool at the end, the usual fixes are a larger pipe, a lower flow, or a shorter run. The diameter table above shows which of these helps most.
Related calculators
The GPM calculator helps find the flow, and the pipe velocity calculator checks that water is not moving too fast. The pipe volume calculator finds the water held in the line. Confirm any design that must meet code with a licensed plumber or engineer.
Frequently asked questions
What is the Hazen-Williams formula used for?
It estimates the pressure a flowing stream of water loses to pipe friction. Irrigation, plumbing, and fire protection designers use it for water at normal temperatures. It does not cover other fluids, very hot water, or very slow flows.
What C-factor should I use for my pipe material?
The C-factor measures smoothness. Plastic pipe and copper sit around 140 to 150, as the form note says. Older steel, galvanized, and cast iron use lower values, because roughness and scale slow the water. Use the value your pipe maker or design manual lists.
Why does friction loss increase so quickly with flow rate?
The formula raises flow to the power 1.852, so doubling the flow multiplies the loss by about 3.6. In this tool, 2-inch pipe at 25 GPM loses 0.64 psi per 100 ft, at 50 GPM it is 2.30, and at 100 GPM it is 8.29.
How much does a larger pipe diameter reduce friction loss?
A lot, because diameter is raised to the power 4.87. At 50 GPM and C 140, a 1.5 in pipe gives 9.32 psi per 100 ft, a 2 in pipe gives 2.30, and a 3 in pipe gives 0.32.
Does this include fittings, elevation, or pump head?
No. It counts straight-pipe friction only. Elbows, valves, and filters add loss, and a rise in elevation adds pressure demand. Add those separately, and have a plumbing or irrigation engineer confirm any system that must meet a code or a performance target.
Estimates only. Verify quantities with your supplier before purchasing.