Pipe Flow Calculator
This tool applies the continuity equation for a full, round pipe: Q = A × v, where Q is the volumetric flow rate (m³/s), A is the cross-sectional area (m²), and v is the mean flow velocity (m/s).
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the pipe's INNER diameter in mm (the bore, not the outside/nominal size).
- Enter either the flow velocity (m/s) or the flow rate (L/min) — the calculator solves for the other.
- Read the flow rate in L/min, L/s and m³/h, plus the velocity and cross-sectional area. If you enter both velocity and flow rate, the flow rate wins and the velocity is recomputed for that pipe size.
How it works
This tool applies the continuity equation for a full, round pipe: Q = A × v, where Q is the volumetric flow rate (m³/s), A is the cross-sectional area (m²), and v is the mean flow velocity (m/s). The area of a circular bore is A = π × (d ÷ 2)², so from the inner diameter d the calculator finds A, then multiplies by velocity to get flow (or divides flow by area to get velocity). Results are converted to L/min, L/s and m³/h for convenience.
The result is the mean (bulk-average) velocity across the pipe — real velocity is higher at the centre and near zero at the wall. The tool assumes a full pipe of circular cross-section flowing under steady conditions and ignores friction/head loss, fittings, elevation and pressure. It does not size a pipe or check that a given velocity is acceptable; typical plumbing design keeps water velocity in a band (often around 1–2.5 m/s for cold-water supply) to limit noise and erosion. Always confirm sizing against the relevant plumbing/building code and a licensed professional.
Worked example
50 mm pipe running at 2 m/s. A pipe with an inner diameter of 50 mm carries water at a velocity of 2 m/s. The cross-sectional area is A = π × (0.025 m)² = 0.0019635 m² (1,963.5 mm²). Using continuity, Q = A × v = 0.0019635 × 2 = 0.0039270 m³/s. Converting: 3.93 L/s, 235.62 L/min, or 14.14 m³/h. To move the same flow through a narrower 25 mm pipe you would need roughly four times the velocity, because area scales with the square of the diameter.
Common mistakes
- Using the nominal or OUTSIDE diameter instead of the inner bore. Wall thickness matters — a '50 mm' pipe may have a bore of 46 mm or less, which changes the area (and flow) noticeably because area depends on diameter squared.
- Mixing up flow-rate units. 235.62 L/min is the same flow as 3.93 L/s and 14.14 m³/h — divide L/min by 60 to get L/s, not the other way around.
- Treating the mean velocity as a real-world design answer. This calculator ignores friction, fittings and head loss, so use it for a first estimate only and check pressure-drop and code limits separately.
Frequently asked questions
Does this include friction or pressure loss?
No. It only relates diameter, velocity and flow through Q = A × v (the continuity equation). Friction losses depend on pipe roughness, length, fittings and the flow regime (see a Reynolds number or Hazen–Williams / Darcy calculation). Treat this as a first-pass sizing estimate, not a pressure-drop result.
Should I use the inner or outer diameter?
Always the inner diameter (the bore). Flow happens through the open bore, and area scales with diameter squared, so using the outside or nominal size overstates the area and the flow. Check the pipe's published bore for its material and pressure class.
What water velocity is normal in a pipe?
For cold-water supply, designers often aim for roughly 1–2.5 m/s to balance pipe size against noise and erosion; hot and drainage systems use different targets. These are general guides only — your local plumbing/building code sets the binding limits, so confirm with the code and a licensed professional.
Can I use it for gases or non-round ducts?
The Q = A × v relationship holds for any fluid, but this tool assumes a full, round pipe. For rectangular ducts or partially filled pipes the area formula differs, and for compressible gas at high speed density changes matter. Use it for full round pipes running liquid for best accuracy.
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- Reynolds Number Calculator
- Water Tank Volume Calculator
- Irrigation Flow Calculator
- Litres per Minute to Cubic Metres per Hour Converter
- Pipe Velocity Calculator
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