Pipe Flow Rate Calculator

How to Use This Calculator

Enter the inside diameter of your pipe in inches and the water velocity in feet per second. If you do not know the velocity, use 5 ft/sec as a common design target. The calculator converts pipe cross-section and velocity into flow rate in GPM, GPH, and CFS.

Why This Matters

Undersized pipe creates excessive friction loss, reducing pressure at sprinklers or emitters and causing uneven irrigation. Oversized pipe wastes money on materials. Correct pipe sizing ensures adequate flow and pressure throughout the entire system.

Methodology

Flow area = π × (Diameter/2)² in square feet. Flow rate (CFS) = Area × Velocity. GPM = CFS × 448.83. GPH = GPM × 60. Recommended velocity is 3-5 ft/sec for PVC to minimize friction loss and water hammer risk.

Common Mistakes to Avoid

• Using nominal pipe size instead of actual inside diameter — a 4-inch PVC pipe has a 4.026-inch ID.
• Designing for velocity above 5 ft/sec in PVC, which increases water hammer and pipe failure risk.
• Not accounting for friction loss over long pipe runs — pressure drops significantly over distance.
• Ignoring elevation changes, which add or subtract 0.433 PSI per foot of elevation difference.