Flow Rate Calculator
Calculate flow rate through a pipe or hose in GPM, CFS, and liters per minute from water velocity and pipe diameter.
How this calculator works
Flow rate through a full, pressurized pipe or hose is simply how fast the water is moving multiplied by the size of the opening it is moving through. This calculator takes your water velocity and pipe diameter, works out the cross-sectional area, and multiplies the two together to get the flow rate.
Formula: Flow rate = velocity × cross-sectional area (Q = VA). Cubic feet per second (CFS) converts to gallons per minute by multiplying by 448.831, and to liters per minute by multiplying by 1,699.01.
Worked example
5 ft/s velocity through a 4-inch diameter pipe:
- Pipe diameter: 4 inches
- Cross-sectional area: ≈ 0.0873 sq ft
- Flow rate: ≈ 0.436 CFS
- Converted to GPM: ≈ 195.84 GPM
- Converted to liters per minute: ≈ 741.33 L/min
Notes
Flow rate scales with the square of diameter, not linearly. A pipe twice as wide carries four times the flow at the same velocity, which is why upsizing a pipe even slightly makes a big difference in capacity.
This calculator covers pressurized, full-pipe or hose flow — garden hoses, sprinkler lines, and water supply pipes all fit this case. For a partially full pipe or an open channel like a ditch or culvert, use the Manning’s Equation Calculator instead, since that situation depends on the wetted portion of the cross-section rather than the full pipe area.
How to use
Enter the water velocity and the inside diameter of the pipe or hose. The calculator works out the cross-sectional area for you and returns flow rate in CFS, GPM, and liters per minute, so you can compare against pump specs, fixture ratings, or irrigation requirements in whichever unit you need.
A planning estimate using standard hydraulics formulas. For code-governed drainage or civil design, confirm with a licensed civil engineer.
Frequently asked questions
What is the formula for flow rate through a pipe?
Flow rate equals velocity times cross-sectional area, written as Q = VA. Water moving at a certain speed through a pipe opening of a certain size gives a volume per unit of time. This calculator handles the area math for you and converts the result into GPM, CFS, and liters per minute.
Why does pipe diameter matter so much to flow rate?
Cross-sectional area grows with the square of diameter, not in a straight line with it. Doubling a pipe's diameter roughly quadruples its area, so at the same water velocity it carries about four times the flow. That is why even a modest increase in pipe size can meaningfully raise capacity.
Does this calculator work for garden hoses and sprinkler lines, not just pipes?
Yes. Any full, pressurized flow through a round opening — a garden hose, a sprinkler supply line, a water main — uses the same Q = VA relationship. Enter the inside diameter of the hose or pipe and the water velocity, and the calculator returns flow rate in your preferred unit.
What if my pipe or channel is not running full?
This calculator assumes the pipe or hose is flowing full under pressure. For a pipe that is only partially full, or an open channel, ditch, or culvert where water has a free surface, use Manning's equation instead, which accounts for the wetted portion of the cross-section and the channel slope.
Estimates only. Verify quantities with your supplier before purchasing.