Pipe flow velocity
The continuity calculation that links a pipe’s inside area, volumetric flow and mean velocity.
ENGINEERING PRINCIPLE
For steady incompressible flow in a full pipe, volumetric flow equals cross-sectional area multiplied by mean velocity.
v = Q/A; A = πD²/4Variables and units
- v = mean velocity (m/s)
- Q = volumetric flow (m³/s)
- D = inside diameter (m)
Worked example
At 18 m³/h through an 80 mm ID pipe, Q = 0.005 m³/s and A = 0.00503 m², giving v = 0.995 m/s.
Calculation method
- Convert flow to m³/s and diameter to metres.
- Calculate the actual internal flow area.
- Divide flow by area, then use the result in a separate pressure-drop review.
Practical applications
- Water distribution
- Process lines
- Preliminary pump-system layout
Review boundary and reference
- Nominal pipe size is not necessarily inside diameter.
- Velocity alone does not indicate pressure loss, cavitation risk or noise.
Formula and displayed units independently checked for the stated conditions. This is not a code-compliance design method.
Source: Continuity equation from fluid mechanics; SI relationships follow NIST Special Publication 811.