Plumbing Pipe Size Calculator
Determine the correct water supply pipe diameter based on your required flow rate and maximum velocity. This calculator uses standard hydraulic formulas to recommend nominal pipe sizes for copper, PEX, and CPVC piping.
Enter Flow Requirements
Full Breakdown
Design Flow Rate
Max Velocity
Min. Inside Diameter
Recommended Nominal Size
Actual Inside Diameter
Actual Velocity
Pipe Material
How to Use This Calculator
- Enter the required flow rate in gallons per minute (GPM), or enter total fixture units to auto-calculate GPM.
- Select the maximum velocity (5 ft/s for copper, 8 ft/s for PEX/CPVC is typical).
- Choose your pipe material.
- Click "Calculate Pipe Size" to get the recommended nominal pipe size.
The Formula
Q (ft³/s) = GPM ÷ 448.83
A (ft²) = Q ÷ Velocity
Inside Diameter (in) = √(4 × A ÷ π) × 12
Select next standard pipe size ≥ calculated diameter
A (ft²) = Q ÷ Velocity
Inside Diameter (in) = √(4 × A ÷ π) × 12
Select next standard pipe size ≥ calculated diameter
Flow rate from fixture units uses the Hunter's Curve approximation. Standard nominal pipe sizes: ½″, ¾″, 1″, 1¼″, 1½″, 2″, 2½″, 3″. Always verify with local plumbing code.