Pipe Flow Rate Calculator
Work out how much water a pipe carries in litres per second and cubic metres per hour.
Result
How it is calculated
The inner diameter d is entered in millimetres and converted to a radius in metres: r = d / 2000. The area is A = π × r² in m², and the flow is Q = A × v in m³/s, where v is the mean velocity in m/s. The result is also shown as Q × 1000 in L/s and Q × 3600 in m³/h. The tool takes the velocity as one average value over the whole cross-section and assumes the pipe runs full. It does not model friction, pressure or fluid type. Zero or negative inputs give no result, and neither does a velocity above 100 m/s, which no real pipe carries. The m³/s and area figures are rounded to four decimals, so for small pipes use the L/s figure for precision.
Worked example
An inner diameter of 50 mm and a mean velocity of 1.5 m/s. Radius = 50 / 2000 = 0.025 m. Area = π × 0.025² = 0.0019635 m², shown as 0.0020 m². Flow = 0.0019635 × 1.5 = 0.0029452 m³/s, shown as 0.0029 m³/s. In litres that is 0.0029452 × 1000 = 2.95 L/s, and per hour 0.0029452 × 3600 = 10.60 m³/h.
Common mistakes
- Entering the outer diameter or the nominal pipe size. The calculation needs the inner diameter, which is smaller because of the wall thickness.
- Entering the diameter in centimetres or metres. The field is in millimetres, so a 5 cm bore is 50.
- Entering the diameter and expecting the radius to be used. Half of that is taken inside the tool, so do not halve it first.
- Using a velocity for a pipe that is not full, such as a drain or gravity line. The result assumes a full cross-section.
- Reading the m³/s figure for a small pipe. It is rounded to four decimals, so a 15 mm pipe at 1 m/s shows 0.0002 and hides detail that the L/s value (0.18) keeps.
About this calculator
The volume of fluid moving through a pipe per unit time — its volumetric flow rate — follows the continuity equation from fluid mechanics: Q = A × v, where Q is flow rate, A is the pipe’s cross-sectional area (calculated from its inner diameter as A = π × (d/2)²), and v is the average flow velocity. Because area scales with the square of the diameter, even a modest increase in pipe diameter dramatically increases the flow rate a pipe can carry at the same velocity.
Plumbers and mechanical/civil engineers use this relationship to size pipework for a required flow — undersized pipe diameter forces higher velocity to deliver the same flow, which increases friction losses, pressure drop and noise, while oversized pipe wastes material and can allow sediment to settle at low velocities. It’s the same equation behind sizing everything from domestic water supply lines to industrial process piping and irrigation systems.
This calculator takes your pipe’s inner diameter and flow velocity and returns the volumetric flow rate in litres per second and cubic metres per hour.
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