Pipe Flow Velocity Calculator

How it is calculated

The tool assumes a full, circular pipe and SI units: the volumetric flow rate Q in cubic metres per second and the inside diameter d in metres. First it finds the cross-section, then the mean velocity:

A = π × (d / 2)²
v = Q ÷ A

It reports v in m/s with four decimals and A in m² with six. Flow can be 0 or more (zero gives zero velocity); the diameter must be above 0. Since the result is an average over the cross-section, the velocity at the pipe wall is lower and at the centre higher. Use the inside diameter, not the outside one, and convert litres per second or gallons per minute to m³/s first (1 L/s = 0.001 m³/s).

Common mistakes

• Entering the diameter in millimetres, such as 50. The field expects metres: 50 gives an area of nearly 2,000 m² and a velocity of 0.0025 m/s. Enter 0.05.
• Entering flow in litres per second or gallons per minute without converting. The flow field is in m³/s, so 5 L/s must be typed as 0.005.
• Using the outside diameter of the pipe. The fluid only occupies the bore, so use the inside diameter, which is smaller by twice the wall thickness.
• Entering the radius where the diameter is expected. Velocity then comes out four times too high, because area depends on the square of the diameter.
• Treating the result as a limit check. It is an average for a full pipe running at steady flow; part-full pipes and fittings are not modelled.