Pipe Flow Rate Calculator
This calculator turns a pipe's inner diameter and the water velocity inside it into a volumetric flow rate. It computes the cross-sectional area as pi times the inner radius squared, then multiplies by the velocity to report gallons per minute and cubic feet per second. Use it to size a line, sanity-check a design velocity, or see how much water a given pipe can carry at a chosen speed.
How to calculate pipe flow rate
- Enter the inner diameter of the pipe, in inches, in the first box.
- Enter the water velocity, in feet per second, in the second box.
- Read the large gallons per minute figure at the top of the result panel.
- Check the three cards for flow area in square inches, GPM, and cubic feet per second.
- Watch for the amber note that appears when velocity is above 8 ft/s.
- Press Copy result to grab the flow rate as a one-line summary.
Examples
1 inch pipe at 5 ft/s
Inner diameter 1 in, velocity 5 ft/s
12.24 gpm, flow area 0.785 sq in, 0.0273 cubic feet per second
2 inch pipe at 6 ft/s
Inner diameter 2 in, velocity 6 ft/s
58.75 gpm, flow area 3.142 sq in, 0.1309 cubic feet per second
4 inch main at 7 ft/s
Inner diameter 4 in, velocity 7 ft/s
274.18 gpm, flow area 12.566 sq in, 0.6109 cubic feet per second
Frequently asked questions
How does the calculator get flow rate from velocity?
It finds the cross-sectional area as pi times the inner radius squared in square inches, divides by 144 to reach square feet, then multiplies by the velocity in feet per second for cubic feet per second. Multiplying that by 7.480519 gallons per cubic foot and 60 seconds gives gallons per minute.
Should I use the inner diameter or the nominal pipe size?
Use the true inner diameter, not the nominal label. A pipe called 1 inch often has an inner bore that differs by wall thickness and schedule, and even a small diameter error changes the area a lot because area follows the square of the diameter.
What velocity is reasonable for a water pipe?
Common plumbing practice keeps water velocity around 5 to 8 feet per second. Above roughly 8 ft/s the tool flags the value, since faster flow raises noise, pipe erosion, and water hammer risk, while very slow flow can let sediment settle out.
Why does a slightly larger pipe carry so much more water?
Flow rises with the cross-sectional area, and area grows with the square of the diameter. Going from a 1 inch to a 2 inch bore at the same velocity roughly quadruples the flow, which is why upsizing a line makes such a large difference.
Is this the same flow my pump or supply will actually deliver?
Not necessarily. Real delivery also depends on supply pressure, friction loss along the run, elevation, and fittings. This tool answers a narrower question: how much flow a chosen velocity represents in a pipe of that size.
How do I convert the GPM result to cubic feet per second myself?
Divide gallons per minute by 448.831, because one cubic foot per second equals 7.480519 gallons times 60 seconds. You do not need to do this by hand, since the tool shows both figures side by side.
Does the tool account for friction or pipe roughness?
No. It assumes you already know the velocity and reports the flow that velocity produces. Estimating the velocity itself from pressure, length, and pipe material is a separate friction-loss calculation.
Can I work backwards from a target flow rate?
Not directly in this tool, but you can adjust the velocity or diameter until the GPM matches your target. Because the relationship is straightforward, a few tries will land you on the diameter and velocity pair you need.
Is my input sent anywhere?
No. The calculation runs entirely in your browser using plain arithmetic, so the numbers you type never leave your device and nothing is stored.
Learn more
- Pipe velocity and flow rate, and how they connect
Why flow rate depends on both pipe size and water speed, how to pick a design velocity, and what happens when a pipe runs too fast or too slow.
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