Flow Rate, Pipe Diameter and Velocity Calculator

How flow rate, diameter and velocity relate

In a pipe running full, flow rate is velocity multiplied by cross-sectional area: Q = v × A. For a circular pipe A = π / 4 × d², so knowing any two of the three gives the third.

Which mode do I need?

• Find velocity: an existing line with known flow and pipe size. Too high a velocity means more pressure loss, noise and erosion.
• Find diameter: sizing a new line from a flow rate and a target velocity. The result is a theoretical inside diameter, so pick the next standard size up.
• Find flow rate: known pipe and measured velocity, looking for the quantity passing.

Example

With 120 cubic metres per hour through a pipe of 150 mm inside diameter, the area is π / 4 × 0.15² = 0.01767 m² and the velocity is (120 / 3600) / 0.01767 = 1.89 m/s.

Note

Use the actual inside diameter, not the nominal size (DN). The bore of a DN 150 steel pipe differs from 150 mm depending on wall thickness. Partly full sewers and open channels are not covered by this formula.