Pipe Flow Rate Calculator
This calculator applies the continuity relation Q = A x v to a round pipe running full. Give it the inside diameter plus either the flow velocity or the volumetric flow rate, and it solves for the value you left out. Alongside the answer it reports the cross-sectional area and converts the flow into litres per minute and US gallons per minute.
How to calculate pipe flow rate
- In the 'I know the' menu, choose Velocity if you have the fluid speed, or Flow rate if you have the volume per second.
- Type the inside pipe diameter in metres into the Pipe diameter field.
- Enter the known value in the field that appears: Velocity in m/s, or Flow rate in m3/s.
- Read the solved value together with the cross-sectional area in the result cards.
- Check the Litres / min and US gallons / min cards for the everyday-unit equivalents.
- Click Copy to grab the area, velocity, flow rate, L/min and GPM as plain text.
Examples
Flow rate from velocity in a 100 mm pipe
diameter = 0.1 m, velocity = 3 m/s
area 0.007854 m2, Q = 0.023562 m3/s (1413.72 L/min, 373.46 GPM)
Smaller 50 mm pipe at a lower speed
diameter = 0.05 m, velocity = 2 m/s
area 0.001963 m2, Q = 0.003927 m3/s (235.62 L/min, 62.24 GPM)
Velocity from a known flow rate
diameter = 0.15 m, flow rate = 0.05 m3/s
velocity = 2.8294 m/s, area 0.017671 m2 (3000.00 L/min, 792.52 GPM)
Frequently asked questions
What is the pipe flow rate formula?
Flow rate Q equals cross-sectional area times velocity, Q = A x v. For a round pipe the area is A = pi x (D / 2) squared, where D is the inside diameter, so a wider pipe or faster flow both raise Q.
What units does the calculator use?
Inputs are SI: metres for diameter, metres per second for velocity, and cubic metres per second for flow rate. The output repeats the flow in litres per minute and US gallons per minute so you can compare against pump and meter ratings.
How do I convert m3/s to GPM and L/min?
Multiply cubic metres per second by 15850.323 for US gallons per minute, or by 60000 for litres per minute. The calculator applies both factors for you on every result.
Should I enter inside or outside diameter?
Use the inside diameter, the bore the fluid actually fills. Nominal pipe sizes and outside diameters include the wall thickness, so plugging them in overstates the area and inflates the flow rate.
Why must the diameter be greater than zero?
The area depends on the diameter squared, so a zero diameter gives zero area. When you supply a flow rate the tool divides by the area to find velocity, and a zero area would be a division by zero, so the calculator rejects it.
Can I enter US gallons per minute or a pipe size in inches directly?
No, the fields expect metres and SI flow units. Convert an inch diameter to metres first (inches times 0.0254), and convert GPM to m3/s (GPM divided by 15850.323) before entering it as the flow rate.
Does this account for friction, pressure or turbulence?
No. It is a pure continuity calculation for a full pipe and assumes steady, uniform flow. Head loss, pressure drop and the effect of the Reynolds number are separate problems handled by other tools.
What happens if I leave a field blank or enter a negative number?
The calculator shows a prompt to enter the diameter and known value, and it treats zero or negative entries as invalid because a physical diameter, velocity and flow rate are all positive quantities.
Is my data sent anywhere?
No. The maths runs entirely in your browser, so the numbers you type never leave your device and nothing is stored on a server.
Learn more
- The pipe flow rate formula, from area to GPM
How Q = A times v works for a round pipe, why inside diameter matters, and how to convert cubic metres per second into L/min and gallons per minute.
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