Pipe Flow Calculator
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 4 inch PVC, 200 ft long, with a C of 150 and 10 ft 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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