GPM Calculator
Find pipe flow rate in GPM: enter water velocity and inside pipe diameter to get gallons per minute, using GPM = velocity × area × 448.831.
How this calculator works
The flow through a pipe is set by two things: how fast the water moves and how much open area it moves through. Multiply the water velocity in feet per second by the pipe’s inside cross-section area in square feet, and you get a flow in cubic feet per second. Multiply that by 448.831 to convert into gallons per minute.
The cross-section area comes from the pipe’s inside diameter. The calculator finds it for you, so you only enter the velocity and the diameter. Because area grows with the square of the diameter, a wider pipe carries far more water at the same speed.
Formula: Flow (GPM) = velocity (ft/s) × pipe cross-section area (sq ft) × 448.831. Area = π × (diameter ÷ 2 ÷ 12)².
Worked example
Water moving at 5 ft/s through a 2-inch pipe:
- Velocity: 5 ft/s
- Cross-section area ≈ 0.0218 sq ft
- Flow = 5 × 0.0218 × 448.831 ≈ 48.96 GPM
Reference
At 5 ft/s:
| Inside diameter | Flow |
|---|---|
| 0.75 in | ≈6.9 GPM |
| 1 in | ≈12.2 GPM |
| 1.5 in | ≈27.5 GPM |
| 2 in | ≈49.0 GPM |
| 3 in | ≈110.2 GPM |
How to use
Measure or look up the pipe’s inside diameter in inches and estimate the water velocity in feet per second. Enter both, and the calculator converts the diameter into a cross-section area and returns the flow in gallons per minute. Use it to check whether a given pipe can carry the flow your fixture or pump needs.
Flow and velocity are planning estimates. Keep supply-line velocity under about 8 ft/s to limit noise and erosion; confirm pipe sizing with local plumbing code.
Frequently asked questions
How do I calculate flow rate in GPM?
Flow depends on how fast the water moves and how wide the pipe is. You multiply the water velocity in feet per second by the pipe's inside cross-section area in square feet, then multiply by 448.831 to convert cubic feet per second into gallons per minute. This calculator handles the area math from the pipe's inside diameter and returns the flow directly, so you only enter velocity and diameter.
Where does the number 448.831 come from?
It is a unit conversion. Velocity times area gives you a flow in cubic feet per second. There are 7.48052 gallons in a cubic foot and 60 seconds in a minute, and 7.48052 times 60 is 448.831. Multiplying by that single constant converts cubic feet per second straight into gallons per minute, which is why it appears in the formula.
Why does inside diameter matter more than pipe size on the label?
Flow depends on the actual open area the water passes through, which is set by the inside diameter, not the nominal size printed on the pipe. Wall thickness and material change how much the inside diameter differs from the label. Because area grows with the square of the diameter, even a small change in the true inside diameter has a large effect on the flow the pipe can carry.
What velocity should I design for?
For water supply lines, plumbers usually keep velocity under about 8 feet per second. Faster water makes noise, accelerates erosion of the pipe wall, and raises pressure loss along the run. If your calculated flow needs a velocity above that range, step up to a larger diameter so the same flow travels through more open area at a gentler speed.
Estimates only. Verify quantities with your supplier before purchasing.