Pipe Flow Rate Calculator
Find the flow rate through a pipe from the water velocity. Enter the inner diameter in inches and the velocity in feet per second to get the flow in gallons per minute and cubic feet per second.
How to calculate pipe flow rate
- Measure the inner pipe diameter in inches.
- Enter the water velocity in feet per second.
- Read the flow rate in gallons per minute and cubic feet per second.
Examples
1 inch pipe at 5 ft/s
1 in inner diameter, 5 ft per second
About 12.24 gallons per minute
Frequently asked questions
How do I find flow rate from velocity?
Multiply the pipe cross-section area by the velocity. Area is pi times the inner radius squared, then convert units to get gallons per minute.
What water velocity is recommended in a pipe?
Most plumbing keeps velocity around 5 to 8 feet per second. Higher speeds raise noise, erosion and water hammer risk, while very low speeds can let sediment settle.
Why does a small pipe limit flow so much?
Flow scales with the area, which grows with the square of the diameter. Doubling the diameter quadruples the area and the flow at the same velocity.
Is this the same as the flow my pump delivers?
Not always. Actual flow also depends on pressure, friction loss and fittings. This gives the flow that a given velocity in a given pipe represents.
How do I convert GPM to cubic feet per second?
Divide gallons per minute by 448.831, since one cubic foot per second equals 7.480519 gallons times 60 seconds. The tool reports both for you.
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