Flow Rate & Pipe Velocity Calculator
Flow from pipe size and velocity, velocity from flow and size, or the diameter you need
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Works out the mean velocity in a pipe or duct from the flow rate, the flow rate from the velocity, or the size needed for a given flow and velocity.
How to use
- Under "Cross-section", choose "Round pipe or duct" or "Rectangular duct", and under "Calculate" pick "Velocity", "Flow rate" or "Size".
- Enter the two known values and choose a unit for each, for example L/min, m³/h, GPM or CFM.
- For a steel pipe, pick it under "Nominal steel pipe" to fill in the inside diameter.
- Open "Working" to see each step. "Time to fill a tank" and the "Flow rate unit converter" are below.
How it works
- Formula: Q = A × V, with A = π × D² ÷ 4 for a round bore and A = W × H for a rectangle. The result is the mean velocity.
- For a rectangular duct the same-area diameter √(4 × W × H ÷ π) and the hydraulic diameter 2 × W × H ÷ (W + H) are shown separately.
- Pipe bores: ASME B36.10M / B36.19M steel pipe, NPS 1/8 to 8, Schedule 40 and 80.
- Geometry only: no pressure loss, recommended velocities or gas and steam design.
Questions and answers
- How do I convert flow rate to velocity?
- Divide the flow by the inside area. 60 L/min (0.001 m³/s) in a 26.64 mm bore (557.4 mm²) gives about 1.79 m/s.
- What pipe size do I need for a given flow?
- Choose "Size" and enter the flow and the velocity you want; the page gives D = √(4 × Q ÷ (π × V)). Pick a pipe with at least that bore.
- What is the difference between CMH, LPM, GPM and CFM?
- CMH is cubic metres per hour, LPM litres per minute, GPM US gallons per minute and CFM cubic feet per minute. 1 m³/h is 16.67 L/min, 4.403 GPM and 0.5886 CFM.
- Why is the hydraulic diameter of a duct smaller than the equivalent diameter?
- It is based on area divided by perimeter, so flat ducts get small values. A 400 × 200 mm duct: same-area diameter about 319 mm, hydraulic diameter about 267 mm.