About the Pipe Flow Rate Calculator
Continuity Equation
The Pipe Flow Rate Calculator computes volumetric discharge (Gallons Per Minute, L/s, CFS) and fluid velocity (ft/s, m/s) based on pipe inside diameter and flow speed.
Plumbing codes recommend maintaining fluid velocity between 4 to 8 ft/s (1.2 to 2.4 m/s). Velocities below 2 ft/s allow solids to settle, while velocities above 8–10 ft/s cause pipe erosion, cavitation, and severe water hammer.
Sizing copper tubing velocity limits? Check our copper pipe flow rate calculator, calculate gravity DWV fall with the drain pipe slope calculator, or calculate fixture load with the DFU calculator.
Hydraulic Continuity & Flow Rate Reference
Volumetric discharge is governed by fluid velocity ($v$) and internal cross-sectional area ($A$):
Schedule 40 PVC & Steel Flow Capacities
| Nominal Size | Inside Dia | Area (in²) | Flow @ 4 ft/s | Flow @ 6 ft/s | Flow @ 8 ft/s |
| 1/2" Sch 40 | 0.622" | 0.304 in² | 3.79 GPM | 5.68 GPM | 7.58 GPM |
| 3/4" Sch 40 | 0.824" | 0.533 in² | 6.65 GPM | 9.97 GPM | 13.30 GPM |
| 1" Sch 40 | 1.049" | 0.864 in² | 10.78 GPM | 16.16 GPM | 21.55 GPM |
| 1-1/2" Sch 40 | 1.610" | 2.036 in² | 25.38 GPM | 38.07 GPM | 50.76 GPM |
| 2" Sch 40 | 2.067" | 3.356 in² | 41.84 GPM | 62.77 GPM | 83.69 GPM |
| 3" Sch 40 | 3.068" | 7.393 in² | 92.17 GPM | 138.25 GPM | 184.34 GPM |
Dedicated Calculators by Flow Rate Size
Real-World Calculation Example
Scenario: Sizing a 1.5-Inch Schedule 40 Irrigation Header
Determining flow capacity for a 1.5-inch Schedule 40 PVC pipe (Inside Diameter = 1.610 inches) at a design velocity of 5.0 ft/s.
• Flow Formula: Q = 2.448 × (1.610)² × 5.0 = 31.73 GPM
• Metric Equivalent: 31.73 × 0.06309 = 2.00 Litres/second
