Pipe Flow Velocity Calculator
Work out how fast water moves in a pipe from the flow and the pipe’s actual inside diameter — or the flow you get at a chosen velocity.
Method reviewed October 26, 2026. Version 1.0.0. How it works
Result
- Velocity
- 3.93ft/s
10 gpm in a 1.02 in. bore moves at 3.93 ft/s.
- Pipe and fitting manufacturers publish velocity limits for noise and erosion that depend on the material and temperature. Check yours; this tool does not apply a limit.
- Flow rate
- 10gpm
- Flow area
- 0.817in²
Show the working
- V (ft/s) = GPM × 231 in³/gal ÷ 60 s ÷ (π/4 × ID²) ÷ 12 = 0.4085 × GPM ÷ ID²0.4085 × 10 ÷ 1.02² = 3.926 ft/s
The formula
Velocity is flow divided by the open area of the pipe. In US units it simplifies to V (ft/s) = 0.4085 × GPM ÷ ID², with the inside diameter in inches. The 0.4085 comes from 231 cubic inches per gallon, 60 seconds per minute and the area of a circle.
Use the actual inside diameter from the pipe or tube manufacturer. Nominal sizes are only labels — a ¾-inch copper tube and a ¾-inch plastic pipe have different bores.
Velocity limits
Too fast, and pipes get noisy and erode; too slow, and air and debris settle out. Pipe and fitting manufacturers and design guides publish limits that depend on the material, water temperature and how many hours the system runs. This calculator reports the velocity and leaves the limit to the guidance that applies to your materials.
Worked example
These are the example values the calculator opens with, and the result the engine calculates for them.
Inputs
- Calculate
- Velocity from flow
- Flow rate
- 10 gpm
- Pipe inside diameter
- 1.025 in
Result
- Velocity
- 3.89 ft/s
- Flow rate
- 10 gpm
- Flow area
- 0.825 in²
10 gpm in a 1.025 in. bore moves at 3.89 ft/s.
Assumptions
- 1 US gallon = 231 in³ (NIST). Full-bore flow.
Limitations
- Uses the diameter you enter; nominal pipe sizes are not converted.
Frequently asked questions
Why use the inside diameter?
Only the bore carries water. Wall thickness differs between pipe types and pressure classes, so the nominal size alone does not determine the velocity.
How do I find the flow for a target velocity?
Switch the calculator to “Flow at a velocity” and enter the velocity and inside diameter.
Does this calculate pressure drop?
No. Pressure drop depends on the pipe material and fittings; use manufacturer data or a friction chart for your pipe type.
Sources
- NIST Guide to the SI (SP 811), Appendix B.9 — conversion factors — National Institute of Standards and Technology. 1 ton of refrigeration = 12,000 BTU/h; 1 BTU/h = 0.2930711 W; 1 atm = 101.325 kPa.
Spotted an error? Report a problem with this calculator — see our corrections policy.