Calculate the flow rate from pipe diameter and flow velocity, or the velocity from the flow rate and diameter, with Q = A × v.
Last updated: November 8, 2026
Cross-sectional area × velocity
The volumetric flow rate (Q) is the volume of fluid passing a cross-section per unit of time: • Q = A × v • For a round pipe, A = π × d² ÷ 4 • v = Q ÷ A Example: water flowing at 2 m/s through a pipe with a 50 mm inner diameter has a cross-section of π × 0.05² ÷ 4 = 0.001963 m². The flow rate is 0.001963 × 2 = 0.003927 m³/s = 3.93 L/s, about 236 litres (62 US gallons) a minute. The calculation uses the average velocity across the pipe. Pressure drop and friction need a separate calculation.
Common questions about flow rate
Q = A × v. With area in m² and velocity in m/s, the flow rate is in m³/s. 1 m³/s = 1,000 L/s.
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In millimetres (1 in = 25.4 mm).
Usually 1–3 m/s in water pipes.