Quick takeaway: For a full circular pipe, flow rate is cross-sectional area multiplied by average velocity: Q = A × v. A 2-inch internal diameter at 5 ft/s gives approximately 48.96 US gallons per minute.
Calculate pipe flow from diameter and velocity
Enter the pipe's internal diameter in inches and its average flow velocity in feet per second. The result is US gallons per minute (GPM). Use measured internal diameter, since a nominal pipe size may differ from the actual bore.
US GPM = π × (d / 24)² × v × 448.831
Worked example for a 2-inch pipe
- Radius = 2 ÷ 2 ÷ 12 = 0.0833333 ft.
- Area = π × radius² = 0.0218166 sq ft.
- Volumetric flow = 0.0218166 × 5 = 0.109083 cubic feet per second.
- Convert to US GPM: 0.109083 × 448.831 ≈ 48.96 GPM.
Doubling the internal diameter at the same velocity multiplies the area and flow by four. A 4-inch internal diameter at 5 ft/s gives approximately 195.84 GPM.
Evidence note: NASA's continuity explanation relates flow to area and velocity. This calculator assumes a full circular cross-section and a known average velocity. It does not infer velocity from pressure or determine pump capacity.
Common Mistakes
- Entering outside diameter or nominal trade size instead of internal diameter.
- Entering inches per second in a field that expects feet per second.
- Treating pressure alone as enough to determine flow. Pipe length, roughness, fittings and system losses also matter.
- Applying full-circle area to a partially full gravity drain.
For a related capacity calculation, use the pipe volume calculator.