Pipe size from flow and velocity
Calculate a circular pipe's internal diameter from a chosen flow and mean velocity. This is a continuity calculation, not a pressure-loss or nominal-size selection. Use internal diameter, not the printed outside diameter or DN label.
Reviewed:
Calculator
Flow, area and diameter
Q [m³/s] = Q [L/min] / 60000; A = Q / v; D [mm] = 1000 × √(4Q / (πv))
The velocity must be positive and suitable for the fluid and project. This tool does not recommend a universal maximum velocity. A calculated diameter is a geometric minimum at the chosen velocity; use the real internal bore of each candidate product to recalculate velocity. Wall thickness can make two pipes with the same outside diameter behave differently.
Example: 50 L/min
50 L/min is 0.0008333 m³/s. At an illustrative 1.0 m/s, D = 32.57 mm; at 1.5 m/s, D = 26.60 mm; at 2.0 m/s, D = 23.03 mm. These are internal diameters, not DN32, DN25 or other nominal product instructions. The selected pipe still needs pressure, noise and operating-condition checks.
What diameter alone cannot establish
Available pressure depends on pipe length, roughness, bends, valves, elevation and the pump or supply. Fluid temperature and viscosity also matter. The continuity equation does not predict those losses or verify potable-water, heating or drainage requirements. Compare a candidate internal bore with a full hydraulic calculation and the manufacturer's pressure/temperature rating.
Internal diameter for 10 L/min
At a chosen mean velocity of 1.5 m/s, 10 L/min requires an internal diameter of 11.89 mm from continuity alone. Change the flow and velocity in the calculator. This is not a nominal pipe size and does not calculate pressure losses.
Internal diameter for 30 L/min
At a chosen mean velocity of 1.5 m/s, 30 L/min requires an internal diameter of 20.60 mm from continuity alone. Change the flow and velocity in the calculator. This is not a nominal pipe size and does not calculate pressure losses.
Flow through a 15, 22 or 32 mm internal bore
Q [L/min] = v [m/s] × π × (Dinternal [mm] / 1000)² / 4 × 60000
At an illustrative velocity of 1 m/s, internal diameters 15, 22 and 32 mm give 10.60, 22.81 and 48.25 L/min respectively. Diameter alone cannot determine flow: enter the mean velocity as well. Nominal or external diameter is not the internal bore. Pressure, length, roughness and fittings require a separate hydraulic assessment.
Calculator — Flow through a 15, 22 or 32 mm internal bore
Worked examples — illustrative inputs
| Q [L/min] | v [m/s] | Dinternal [mm] |
|---|---|---|
| 50 | 1.0 | 32.57 |
| 50 | 1.5 | 26.60 |
| 50 | 2.0 | 23.03 |