Field calculator
Pipe flow velocity calculator (GPM to ft/s)
Water velocity is the number that decides whether a pipe is sized right, and it falls out of the flow and the bore. The velocity in feet per second equals 0.4085 times the flow in gallons per minute divided by the inside diameter in inches squared. Enter the flow and the inside diameter to get the velocity. Velocity matters in both directions: too slow and sediment settles and the line fouls, too fast and you get noise, erosion of the pipe wall, and water hammer that can split fittings. Most systems are designed to roughly 2 to 8 feet per second, with tighter limits where quiet operation or erosion resistance matters, often around 5 feet per second for cold water and 3 for hot in copper. Use the actual inside diameter for the pipe material and schedule, not the nominal size, and confirm the velocity limit against the plumbing or piping code, the manufacturer, and the application.
Result
Pipe flow velocity from flow rate: V (ft/s) = 0.4085 × GPM / diameter squared (inside diameter in inches). Enter the flow in gallons per minute and the inside diameter. Velocity matters because too slow lets sediment settle and too fast causes noise, erosion, and water hammer. Typical design limits run about 2 to 8 ft/s, with tighter limits on quiet or erosion-sensitive systems (often around 5 ft/s for cold and 3 ft/s for hot copper). Use the actual inside diameter for the pipe material and schedule, and confirm the velocity limit against the plumbing or piping code, the manufacturer, and the application.
anvilfield.com/calculators/pipe-flow-velocity-calculator · Free field calculators and FieldOS. A planning estimate, verify against the code, the manufacturer, and the engineer of record.
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Pipe velocity FAQ
What is a water supply fixture unit?
A water supply fixture unit, or WSFU, is a dimensionless number assigned to a fixture that represents its load on the water system, folding together how much it draws, how long, and how often. Totaling WSFU and reading the demand curve gives a probable peak flow in gpm. Verify the values against the adopted code.
How do you size water supply pipe?
Total the WSFU for each segment, convert to a probable demand in gpm off the Hunter curve, then build a pressure budget by subtracting elevation, meter, backflow, and fixture residual from the source pressure. Size each segment to carry its demand under both the leftover friction budget and the velocity limit. The adopted code controls.
What velocity is too high for water pipe?
Common design limits are about 8 ft per second for cold water and 5 ft per second for hot, with hot lower because it erodes copper faster. Above 140 degrees F, drop copper to 2 to 3 ft per second. Past these limits you get erosion corrosion, pinholes at the elbows, noise, and water hammer.
Why not size water pipe for every fixture running at once?
Because that moment never happens, and sizing for it builds oversized pipe that then sits stagnant. Not every fixture draws at the same second, and the odds fall as the building grows. That diversity is built into the demand curve, which flattens as fixture units climb, so the curve, not the summed flow, is the demand.
How much pressure do I lose going up a building?
About 0.43 psi for every foot of height, the static head, with a precise figure of 0.433 psi per foot. A fixture 40 ft up has lost roughly 17 psi to elevation before any flow starts. On a high-rise the elevation alone can outrun the street pressure, which is why tall buildings need pressure zones and booster pumps.