Pipe Flow Rate Calculator
Find volumetric flow rate in a pipe.
Inside diameter of the pipe.
Results update as you type.
How it works
The pipe flow rate is the volume of fluid passing through a pipe each second, and this pipe flow rate calculator computes it from the flow velocity and the pipe's inside diameter, giving results in both cubic metres per second and litres per second. Flow rate combines how fast the fluid moves with how wide the pipe is, since a wider pipe carries far more fluid at the same velocity than a narrow one. This matters throughout plumbing and process engineering: water supply, irrigation, HVAC, chemical plants, and fuel systems all specify flow rates to ensure enough fluid reaches its destination without excessive velocity that causes noise, erosion, or pressure loss. A pipe flow rate calculator lets you check that a chosen pipe size and velocity deliver the required throughput, or work backwards to find the velocity a target flow demands. The formula works because the flow rate is simply the cross-sectional area of the pipe multiplied by how fast the fluid crosses it. For a circular pipe the area is pi times the radius squared, and the radius is half the diameter you enter. Multiplying by a thousand restates the volume in litres. Mechanical, civil, and chemical engineers along with plumbers use a pipe flow rate calculator to size pipes and balance systems.
Q = velocity × π × (diameter ÷ 2)²
Volumetric flow rate equals the flow velocity multiplied by the pipe's cross-sectional area. For a round pipe that area is pi times the radius squared, and the radius is half the diameter, so the flow rate is velocity times pi times the diameter over two squared. Multiplying the cubic-metres-per-second result by one thousand converts it to litres per second, since one cubic metre contains a thousand litres.
Source:Standard fluid mechanics (continuity, Q = vA)
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter flow velocity and pipe diameter.
- 2
Adjust the options
Fine-tune any options to match your situation.
- 3
Read your result
Flow rate updates instantly as you type — no button to press.
Frequently asked questions
How is pipe flow rate calculated?
Volumetric flow rate equals the fluid's velocity multiplied by the pipe's cross-sectional area. For a round pipe, the area is pi times the radius squared, where the radius is half the diameter. So flow rate equals velocity times pi times the square of half the diameter. A faster velocity or a wider pipe both increase the volume of fluid delivered each second.
What is the difference between flow rate and velocity?
Velocity is how fast the fluid moves along the pipe, in metres per second, while flow rate is the volume delivered per second, which also depends on the pipe's cross-sectional area. The same velocity in a wide pipe gives a much higher flow rate than in a narrow one. Flow rate is what matters for filling tanks and supplying demand; velocity governs pressure loss and erosion.
Why does pipe diameter have such a big effect?
Because the cross-sectional area depends on the diameter squared, flow rate rises with the square of the diameter at a fixed velocity. Doubling the pipe diameter quadruples the flow it carries. This strong dependence is why even a modest increase in pipe size can dramatically boost capacity, and why undersized pipes force high velocities that waste pressure and cause noise.
What units does this calculator use?
Enter velocity in metres per second and diameter in metres. The primary result is in cubic metres per second, and the calculator also shows litres per second by multiplying by one thousand, since one cubic metre equals a thousand litres. If your diameter is in millimetres, divide by 1000 first. For flow in cubic metres per hour, multiply the result by 3600.
Does this assume the pipe is running full?
Yes. The calculation uses the full cross-sectional area of the pipe, so it assumes the pipe is completely filled with fluid moving at the entered velocity, as in a pressurised system. For a partially filled pipe, such as a gravity drain running part full, only the wetted area carries flow, so you would need the actual filled area rather than the full circular area.