Pipe Flow Velocity Calculator
Enter two of flow rate, inside diameter and velocity. You get the third and whether the flow is smooth or turbulent.
Fill in two of flow rate, diameter and velocity and leave the one you want to find empty. The answer appears as you type, in the unit shown next to its box. Use the inside diameter, not the outside.
Worked examples
Water supply
1 L/s in a 50 mm pipe
0.509 m/s
0.001 / 0.0019635.
Sizing
2 L/s at 1 m/s
50.5 mm bore
sqrt(4 x 0.002 / (pi x 1)).
About Pipe Flow Velocity Calculator
The flow rate through a pipe is the velocity of the fluid multiplied by the cross-section area of the bore. A flow of one litre per second through a pipe 50 millimetres across moves at about half a metre per second. Knowing any two of flow rate, diameter and velocity gives the third, which helps to size pipes for a given flow without making them noisy or wearing them out.
For water systems, designers often keep velocities between 0.5 and 2 metres per second, because faster flow causes noise, pressure loss and erosion, and slower flow can let sediment settle. This calculator also gives the Reynolds number for water at 20 degrees Celsius, which tells you whether the flow is smooth (laminar, below about 2,300) or turbulent (above about 4,000). Most pipework runs turbulent, which is normal. Flow is also shown in litres per minute and cubic metres per hour.
How to use Pipe Flow Velocity Calculator
- Enter any two of flow rate, inside diameter and velocity.
- Choose units such as litres per minute or cubic metres per hour.
- Leave the value you want to find empty.
- Read the answer, the Reynolds number and the type of flow.
Common uses
- Sizing a water supply or heating pipe for a flow rate
- Checking the velocity in an existing pipe
- Choosing a hose or nozzle size
- Estimating when flow will become turbulent
Frequently asked questions
What is the flow rate formula?
Flow rate Q = velocity x area, where the area is pi x diameter squared / 4. A 50 millimetre bore at 0.5093 metres per second carries 1 litre per second.
What velocity is normal in water pipes?
Typically 0.5 to 2 metres per second. Above that, noise and erosion grow and pressure loss rises quickly.
What diameter should I use?
The inside diameter of the bore, not the outside or the nominal size. Wall thickness makes the inside smaller.
What is the Reynolds number?
It compares inertia to viscosity: velocity x diameter / kinematic viscosity. Below about 2,300 flow is laminar and above 4,000 it is turbulent.