Enter the flow rate and the inner diameter. The calculator gives the velocity and the diameter needed for your target velocity.
Flow velocity
m/s
Flow rate
l/s
Diameter for target velocity
mm
Cross-section
cm²
How the velocity is calculated
The mean flow velocity is the volume flow divided by the inside cross-section of the pipe:
v = Q ÷ (π ÷ 4 · D²)
D = √(4 Q ÷ (π · v))
With Q in m³/s, D in metres and v in m/s. Example: 20 m³/h in a DN65 steel pipe with 70.3 mm inside diameter flows at 1.43 m/s. For 1.5 m/s you would need 68.7 mm, so DN65 fits.
Recommended velocities
Application
Velocity (m/s)
Pump suction line, water
0.5 – 1.2
Pump discharge, water
1.0 – 2.5
Building water supply
1.0 – 2.0
Heating circuits
0.3 – 1.0
Viscous oils
0.5 – 1.0
Slurries (to keep solids moving)
1.5 – 3.0
Compressed air mains
6 – 10
Saturated steam
20 – 35
Typical design ranges. Codes and project specifications can set different limits.
Inside diameters of common steel pipes
Nominal size
Outside × wall (mm)
Inside (mm)
DN25
33.7 × 2.6
28.5
DN50
60.3 × 2.9
54.5
DN65
76.1 × 2.9
70.3
DN80
88.9 × 3.2
82.5
DN100
114.3 × 3.6
107.1
DN150
168.3 × 4.5
159.3
DN200
219.1 × 6.3
206.5
Wall thicknesses vary by standard and pressure class. Use the actual inside diameter of your pipe.
Questions and answers
How do I calculate the velocity in a pipe?
Divide the volume flow by the pipe's cross-section: v = Q ÷ (π/4 · D²). With Q in m³/s and D in metres you get m/s. For Q in m³/h, divide by 3,600 first.
What is a good velocity for water pipes?
Pump suction lines 0.5 to 1.2 m/s, pressure lines 1 to 2.5 m/s, gravity drains 0.6 to 1.5 m/s. Building water supply often uses 1 to 2 m/s to keep noise low.
How do I choose the pipe size?
Calculate the diameter for your target velocity, then pick the next standard pipe with an inside diameter at least that large. Check the pressure drop over the full length before you decide.