Pipe Size Calculator
Working Backward from Flow to Diameter
Most pipe-sizing questions start with a known demand — a fixture, an irrigation zone, a piece of equipment — that needs a specific flow rate delivered without exceeding a safe velocity. This calculator rearranges the continuity equation to solve directly for the minimum pipe diameter that keeps velocity at or below a target, rather than making you guess a diameter and check it.
The Formula
A = Q(ft³/s) / Max Velocity
Flow rate is entered in GPM and converted to cubic feet per second, then divided by the target maximum velocity to get required cross-sectional area, which is finally converted to a diameter in inches.
Where This Calculation Matters
- Fixture branch sizing — determining the minimum pipe diameter that delivers a fixture's rated flow without excess velocity.
- Water hammer prevention — keeping velocity under a safe threshold (commonly around 5–8 ft/s for supply lines) reduces the pressure spikes that cause hammer when valves close quickly.
- Retrofit and remodel planning — checking whether an existing pipe diameter is adequate for a new fixture's flow demand.
- Irrigation and agricultural systems — sizing mainlines and laterals to deliver design flow without excessive friction or erosion.
Minimum Diameter at 5 ft/s Design Velocity
| Flow rate | Minimum diameter |
|---|---|
| 10 GPM | 0.90 in |
| 20 GPM | 1.28 in |
| 50 GPM | 2.02 in |
| 100 GPM | 2.86 in |
| 200 GPM | 4.04 in |
Because area scales with the square of diameter, doubling the flow rate only requires about a 41% increase in diameter, not a doubling.
How to Use This Calculator
- Enter the required flow rate in gallons per minute.
- Optionally adjust the maximum velocity in feet per second (defaults to 5).
- Select Calculate to get the minimum pipe diameter needed.
Related Calculations
Once you have a diameter, verify actual flow capacity with the Pipe Flow Calculator, or check the resulting friction loss with the Pipe Pressure Drop Calculator.