Flow Rate Calculator
Get the volumetric flow rate through a round pipe from its inside diameter (mm, inches, or m) and the flow velocity (m/s or ft/s). The tool returns Q = A×v in L/min, US gallons per minute, m³/h, and cubic feet per minute.
Example: with Inside diameter 50 · Diameter unit mm · Flow velocity 2 · Velocity unit m/s → Flow rate: 235.62 L/min.
- US gallons per minute62.24 GPM
- Cubic meters per hour14.14 m³/h
- Cubic feet per minute8.32 CFM
Computed by the calculator below using its default values. Change any input to see your own numbers.
Volumetric flow is the pipe's cross-section times how fast the fluid moves through it: Q = A·v. Widen the pipe and Q grows with the square of the diameter.
Why flow scales with diameter squared
Volumetric flow rate answers a simple question: how much fluid passes a point each minute. For a full pipe it is the cross-sectional area times the average velocity, Q = A·v. The area of a round pipe is π/4·D², so doubling the inside diameter quadruples the area — and the flow — at the same velocity. That squared relationship is why a small bump in pipe size makes such a large difference to capacity.
The catch is that pushing more flow through a fixed pipe means raising the velocity, and pressure loss climbs with roughly the square of velocity. Designers usually keep water below about 5 to 8 ft/s to limit noise, erosion, and pumping cost, then size the pipe to carry the flow within that speed.
How it’s calculated
Q = A·v with A = π/4·D². Diameter converts to meters (inches ×0.0254, mm /1000); velocity to m/s (ft/s ×0.3048). The base result Q (m³/s) scales to units: L/min ×60000, US GPM ×15850.323, m³/h ×3600, CFM ×2118.880.
Assumes a completely full round pipe and uniform average velocity. Real velocity profiles peak at the center, and partly full pipes carry less than this.
Flow at 2 m/s by pipe size
| Inside diameter | Area | Flow (L/min) | Flow (GPM) |
|---|---|---|---|
| 15 mm | 1.77 cm² | 21.2 | 5.6 |
| 25 mm | 4.91 cm² | 58.9 | 15.6 |
| 50 mm | 19.6 cm² | 236 | 62.3 |
| 100 mm | 78.5 cm² | 942 | 249 |
| 150 mm | 177 cm² | 2,121 | 560 |
Computed with Q = π/4·D²·v at v = 2 m/s; rounded.
Common mistakes
- Using the nominal pipe size instead of the true inside diameter — schedule and wall thickness matter.
- Forgetting to convert diameter units; a diameter in mm left as meters is off by a million-fold in area.
- Assuming the pipe runs full — a gravity drain flowing half full carries much less than Q = A·v suggests.
- Mixing up US gallons and Imperial gallons; this tool reports US GPM.
Frequently asked questions
What is the flow rate formula?
Q = A × v, where A is the pipe's cross-sectional area (π/4 × diameter²) and v is the average velocity. With diameter in meters and velocity in m/s, Q comes out in cubic meters per second.
How do I convert m³/s to GPM?
Multiply cubic meters per second by 15,850.3 to get US gallons per minute. This tool does it for you along with L/min, m³/h, and CFM.
Does doubling the pipe diameter double the flow?
No — at the same velocity it quadruples the flow, because area grows with the square of diameter. A 4-inch pipe carries four times a 2-inch pipe, not twice.
What velocity should I design for?
For water, engineers usually keep velocity under about 5 to 8 ft/s (1.5 to 2.4 m/s) to limit noise, erosion, and pressure loss. Size the pipe so the required flow stays in that band.