Pipe size for 30 L/min: internal diameter from velocity
For 30 L/min through a full circular pipe, an assumed mean velocity of 1 m/s requires an internal diameter of about 25.23 mm. At 2 m/s it is about 17.84 mm. These are continuity calculations using your selected velocity; they do not select a nominal commercial pipe size.
Calculate with your assumptions
Calculate the internal diameter
Use Q = A × v and A = πd²/4, so d = √(4Q/(πv)). Here 30 L/min = 0.0005 m³/s. Enter mean velocity in m/s and multiply the diameter in metres by 1000 for millimetres. The flow is treated as steady and the circular pipe as full.
Internal, external and nominal size
The calculated diameter is the inside diameter carrying the water. External diameter, wall thickness and nominal designations are different. Obtain the actual internal dimension from the selected pipe type and wall-thickness series. Two pipes with the same nominal designation may have different bores.
Choose velocity for the application
A velocity input must come from the applicable design basis and material guidance. Changing it changes both the required bore and the hydraulic consequences. The two worked velocities are illustrative; they are not universal limits for cold water, hot water, plastic pipe or copper tube.
Check pressure and the complete route
Actual sizing also depends on available pressure, required pressure at the outlet, pipe length, elevation, fittings, friction, water temperature and the simultaneous demand. The Copper Tube Handbook discusses the relationship among flow, velocity, friction loss and tube size. A bore that meets the velocity calculation may still fail the pressure check.
Examples with stated assumptions
| Inputs and method | Result |
|---|---|
| 30 L/min at 1 m/s | 25.23 mm internal |
| 30 L/min at 1.5 m/s | 20.60 mm internal |
| 30 L/min at 2 m/s | 17.84 mm internal |
Result scope
This is a velocity-based starting point. It excludes friction loss, fittings and pressure availability and does not choose DN, external diameter or a pipe product.