About the Pipe Flow Calculator
The flow in a full pipe is the area of its cross-section multiplied by the speed of the water. This calculator uses that to give the flow in litres per second, cubic metres per hour and US gallons per minute from a pipe's inside diameter and the water speed. It can also work backwards to find the diameter you need for a given flow. Type the inside diameter and speed, or the flow and speed and the pipe flow appears at once, with a few related figures underneath, and you can copy the result with one tap. The sums run in your browser, so nothing you enter is uploaded.
Updated October 2026
How to use the Pipe Flow Calculator
- 1Enter the inside diameter and speed, or the flow and speed.
- 2Pick the units or options from the menus if the pipe flow tool offers them.
- 3Read the pipe flow in the large result box.
- 4Look at the extra rows for related figures.
- 5Press Copy result to keep the answer.
When you need the pipe flow
Plumbers, irrigation planners and students use it to size pipes.
A speed of 1 to 2 m/s is typical for water in buildings. Higher speeds raise friction and noise, while very low speeds let dirt settle.
How the pipe flow is worked out
Flow = π × diameter² ÷ 4 × speed. The diameter for a flow is √(4 × flow ÷ (π × speed)). A 50 mm pipe with water at 1.5 m/s carries 2.95 litres per second, which is 10.6 m³ an hour or 46.7 US gallons a minute. To carry 5 L/s at 1.5 m/s you need about 65 mm.
Every answer on this page comes from that one rule, so you can check any result by hand with a calculator. The figures shown underneath, such as other units, are conversions of the same answer.
- Both directions
- Three flow units
- Inside diameter input
Mistakes to avoid with pipe flow
Use the inside diameter, which is smaller than the nominal size on the label. The result does not include friction losses or pressure; it only links size, speed and flow.
On phone and computer
On a phone, type into the boxes one after another and read the pipe flow underneath; Copy result puts it on the clipboard for a message or a note. On a computer the inputs sit on the left and the answer on the right, so you can change one figure and watch the pipe flow move. The pipe flow calculator works the same on both.
Checking your pipe flow by hand
A good habit is to test the pipe flow calculator with a case you already know. A 50 mm pipe with water at 1.5 m/s carries 2.95 litres per second, which is 10.6 m³ an hour or 46.7 US gallons a minute. To carry 5 L/s at 1.5 m/s you need about 65 mm.
If your own figures give a surprising answer, repeat the working yourself: Flow = π × diameter² ÷ 4 × speed. The diameter for a flow is √(4 × flow ÷ (π × speed)). Small differences usually come from rounding, from mixing units, or from a typing slip in one of the inputs. Flow is only part of a pumping plan. Find the power needed with the Pump Power Calculator, convert units with the Pressure Converter, and compare other flow-driven power with the Wind Turbine Power Calculator.
Features
- Use inside diameter
- Typical speed 1 to 2 m/s
- No friction included
- Clear messages when the pipe flow inputs make no sense
- Copy the result with one tap
- Private: nothing is uploaded
Frequently asked questions
How do I calculate pipe flow?
Flow = π × diameter² ÷ 4 × speed. The diameter for a flow is √(4 × flow ÷ (π × speed)). A 50 mm pipe with water at 1.5 m/s carries 2.95 litres per second, which is 10.6 m³ an hour or 46.7 US gallons a minute. To carry 5 L/s at 1.5 m/s you need about 65 mm.
What is the formula for pipe flow?
Flow = π × diameter² ÷ 4 × speed. The diameter for a flow is √(4 × flow ÷ (π × speed)). The pipe flow calculator applies it exactly and rounds only the displayed answer to six significant figures.
What speed should water have in a pipe?
About 1 to 2 m/s in most building systems.
Is the nominal size the inside diameter?
Often not. Check the pipe's inside diameter.
Does it work for air?
The geometry does, but gases change density with pressure.
Is this pipe flow calculator free and private?
Yes. It is free, needs no account and runs in your browser, so the numbers you type are not uploaded or stored.