Pipe Flow Rate Calculator
Flow rate through a pipe depends on both how fast the fluid moves and how much cross-sectional area it has to move through — the same velocity through a wider pipe delivers far more volume per second.
Inputs
Result
15.708 L/s
56.549 m³/h
How the pipe flow rate calculator works
Cross-sectional area = π × (diameter/2)², converting diameter from mm to metres.
Flow rate = area × velocity, converted to litres per second and cubic metres per hour for readability.
Worked example: 100 mm inner diameter, 2 m/s flow velocity
- Radius in metres: 0.1 ÷ 2 = 0.05 m.
- Area = π × 0.05² = π × 0.0025 ≈ 0.007854 m².
- Flow rate = 0.007854 × 2 = 0.015708 m³/s = 15.708 L/s = 56.55 m³/h.
Common mistakes to avoid
Using outer diameter instead of inner diameter
Flow area depends on the actual internal bore the fluid passes through, not the pipe's outer dimension — pipe walls have thickness, so outer diameter overstates the usable cross-section.
Assuming flow rate doubles when diameter doubles
Because area depends on the square of the radius, doubling the diameter actually quadruples the flow rate at the same velocity, not doubles it — a common and costly sizing mistake.
Frequently asked questions
Why does doubling pipe diameter more than double the flow rate?
Cross-sectional area scales with the square of the radius, so doubling diameter quadruples area — and flow rate scales directly with area at a given velocity.
How do I find required pipe diameter for a target flow rate?
Rearrange: diameter = 2 × √(flow rate ÷ (π × velocity)) — decide on a reasonable velocity for the application first, since velocity itself affects pressure loss and pipe wear.
Does this account for friction losses along the pipe length?
No — this is the ideal flow-rate-from-velocity relationship. Real systems also need pressure-drop calculations that account for pipe length, roughness, and fittings.
What's a typical safe flow velocity for water pipes?
This varies by application and pipe material, but many general guidelines suggest keeping velocity under roughly 2-3 m/s to limit erosion and noise — check relevant codes for your specific use case.
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