Pipe flow calculator: internal diameter and velocity
Flow through a full circular pipe equals its internal cross-sectional area times mean fluid velocity. Enter the actual internal diameter and a stated mean velocity. This geometric calculation does not predict velocity from supply pressure or pipe length.
Calculate with your inputs
Flow formula and units
A = πD² / 4; Q = A × v; L/min = Q[m³/s] × 60000; m³/h = Q[m³/s] × 3600
Convert diameter to metres and velocity to metres per second first. One inch is exactly 0.0254 m and one foot 0.3048 m. A US liquid gallon is 3.785411784 litres. Use unobstructed internal bore, not outside diameter or nominal connection size. The calculation assumes a full circular cross section.
Worked example: 20 mm at 1.5 m/s
A 20 mm diameter has internal area 0.000314159 m². At mean velocity 1.5 m/s, flow is 0.000471239 m³/s, approximately 28.274 L/min, 1.696 m³/h or 7.469 US gallons/min. This is a scenario at the supplied velocity, not a guaranteed capacity of any 20 mm pipe.
What determines actual velocity?
Available pressure, length, roughness, bends, fittings, fluid properties and other losses determine the operating point. Assess required flow together with velocity and pressure loss when choosing pipes, pumps and valves. A larger internal diameter lowers mean velocity at the same flow. This tool does not recommend a universal maximum velocity.
Check the installed product
Read internal dimensions from exact product data and use a measurement or justified operating estimate for velocity. Nominal sizes and wall thickness vary. The full-circle equation does not describe partially filled pipes. The linked pipe-sizing guide reverses the relation for required flow and chosen velocity. Keep units, dimensions and assumptions with each saved result.
Worked examples
| Inputs and method | Result |
|---|---|
| D = 20 mm, v = 1.5 m/s | 28.274 L/min |
| Same flow × 60 / 1000 | 1.696 m³/h |
| 28.274 / 3.785411784 | 7.469 US gal/min |
Result scope
This calculates geometric flow at your supplied velocity. It does not calculate pressure loss, design drainage, assess partially filled conduits or guarantee installed flow. Validate the complete hydraulic design separately.