Pipe Flow Calculator (Metric)
Enter your pipe diameter, length, C factor, and head loss to estimate the water flow in gallons per minute.
What your result means
This is the Hazen-Williams estimate of water flow for a given pipe and pressure drop. The slope is head loss over length. C factors are the pipe smoothness: 150 for PVC, 130 for copper, 100 for steel or cast iron. It's tuned for water at room temperature; for other fluids or pressurized friction-factor work, use the Darcy-Weisbach method.
How to use this calculator
- Enter pipe diameter (use inside diameter).
- Enter pipe length.
- Pick C factor: PVC 150, copper 130, steel 100, cast iron 100.
- Enter head loss (or pressure drop) over the length.
- Read flow in gallons per minute.
The formula
The Hazen-Williams equation: flow equals 0.442 times the C factor times the diameter to the 2.63 power times the slope to the 0.54 power, where slope is head loss over length.
Worked example
Say you've got 10.2 cm PVC, 61 m long, with a C of 150 and 3 m of head loss. The slope is 0.05, and the flow works out to about 504 GPM.
Hazen-Williams C factors
| Material | C factor |
|---|---|
| PVC / plastic | 150 |
| Copper | 130 |
| Concrete | 130 |
| Steel / cast iron | 100 |
Tips & gotchas
- The Hazen-Williams formula is Q = 0.442 * C * D^2.63 * S^0.54.
- C factors: PVC 150, copper 130, steel 100, cast iron 100, concrete 130.
- It's for water at room temperature.
- Slope is head loss divided by length.
- For pressurized systems, friction-factor methods are more precise.
Frequently asked questions
What is the Hazen-Williams equation?
An empirical formula for water flow in pressurized pipes.
What is the C factor?
A friction coefficient by material. Smoother pipe means a higher C.
How accurate is Hazen-Williams?
Good for water at room temp. Use Darcy-Weisbach for other fluids.
What is head loss?
The pressure drop along the pipe, expressed in equivalent feet of water.
How is flow rate measured?
GPM in plumbing; cubic feet per second for large pipes.
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