Flow Rate Calculator
This flow rate calculator finds how much fluid moves through a pipe using the flow rate formula, Q = A × v. Enter the pipe bore and the velocity and the flow rate calculator returns the volumetric flow in liters per minute, gallons per minute, and liters per second. It works as a volumetric flow rate calculator and a pipe flow calculator.
Results
The Flow Rate Calculator is part of our Fluids collection. Browse more fluids to size, convert, and check your work alongside this tool.
What the flow rate calculator does
The flow rate calculator measures how much fluid passes a point each minute, the volumetric flow. It comes from two things: how wide the pipe is and how fast the fluid travels. As a volumetric flow rate calculator and pipe flow calculator, this tool applies the flow rate formula to tell you whether a line can feed a shower, fill a tank in time, or keep a machine supplied.
Flow rate formula
The flow rate formula is the pipe's cross-section multiplied by the velocity:
A is the area of the bore and v is the velocity. This volumetric flow formula is the heart of the flow rate calculator, which converts the millimeter bore and meter-per-second speed into practical volume units and lets you calculate GPM directly.
Flow rate calculator example
A 25 mm pipe with water moving at 2 m/s, through the flow rate formula:
That is about 15.6 gallons per minute from the volumetric flow rate calculator.
How to use the flow rate calculator
Enter the inside diameter of the pipe in millimeters and the flow velocity in meters per second. The flow rate calculator returns the rate in three units so you can read whichever your job speaks, and it doubles as a way to calculate gallons per minute for US work.
Reading the flow rate calculator result
Compare the rate to what the outlet needs. A shower wants roughly 8 to 12 liters a minute; a garden tap more. If the flow rate falls short, either the pipe is too narrow or the pressure driving the velocity is too low. Watch the velocity too: above about 2 m/s pipes get noisy and wear faster, a point the pipe flow calculator helps you check.
When to use the flow rate calculator
- Checking a supply line feeds an outlet with the pipe flow calculator
- Applying the flow rate formula to size pipe for a pump
- Working out how to calculate gallons per minute for irrigation
- Estimating tank fill times with the water flow rate calculator
Common flow rate mistakes
- Using outside diameter. Only the bore carries fluid; use the inside measurement.
- Chasing flow with speed. High velocity brings noise and wear; widen the pipe instead.
- Ignoring fittings. Bends and valves drop the real flow below the flow rate formula figure.
Tips for accurate flow rate results
- Keep velocity under about 2 m/s to avoid noise and wear.
- A slightly wider pipe boosts flow more than you would guess.
- Use the GPM output to calculate gallons per minute for US work.
Why the flow rate formula is not linear in diameter
The relationship between bore and flow surprises people because it is not linear. The flow rate formula depends on the area of the pipe, and area grows with the square of the diameter, so widening a pipe pays off far more than it looks. Step from a 15 mm bore to a 22 mm one, only seven millimeters wider, and the area more than doubles. That single fact, which the flow rate calculator makes concrete, explains why a slightly bigger pipe can transform a weak outlet, and why pinching a line down even a little chokes it hard.
Velocity limits and the pipe flow calculator
Velocity is the other half of the volumetric flow formula, and it carries its own limits. Faster fluid means more flow, but speed above roughly two meters per second brings problems: the rush of water makes pipes sing, sudden valve closures cause water hammer, and the moving fluid scours the pipe walls. Good design keeps velocity in a sensible band and reaches for a wider pipe rather than faster flow when more volume is needed. Running the pipe flow calculator lets you see both levers at once and choose the calm option.
Flow rate reference chart
Flow at common bores and a 2 m/s velocity, from the volumetric flow rate calculator:
| Pipe bore | L/min | GPM |
|---|---|---|
| 15 mm | 21 | 5.6 |
| 25 mm | 59 | 15.6 |
| 50 mm | 236 | 62.3 |
Flow Rate Calculator FAQ
What is the flow rate formula?
The flow rate formula is Q = A × v, the pipe cross-section times the velocity. The flow rate calculator applies this volumetric flow formula and converts the result to L/min, GPM, and L/s.
How do I calculate gallons per minute?
Find the volumetric flow with the flow rate formula, then convert to US gallons. The flow rate calculator shows GPM automatically, so you can calculate GPM without a separate step.
Do I use inside or outside diameter?
Inside diameter. The fluid only flows through the bore, so the outer size would overstate the rate. The pipe flow calculator needs the true bore.
What velocity is normal?
Household pipes usually run 1 to 2 meters per second. Push past 2 and you get noise, water hammer, and faster wear, which the pipe flow rate calculator helps you avoid.