Pipe Diameter Calculator
Calculate required pipe diameter from flow rate and velocity for any pipe material.
Results
Visualization
How It Works
Pipe diameter is determined by the required flow rate and acceptable fluid velocity. Larger pipes reduce velocity and friction losses but cost more. The goal is to balance cost, noise, and pressure drop.
The Formula
D = sqrt(4Q / (pi x V)) where Q is in ft3/s and V is in ft/s
Variables
- D — Pipe inside diameter (inches)
- Q — Flow rate (GPM, converted to ft3/s by dividing by 448.83)
- V — Target velocity (ft/s)
Worked Example
For 10 GPM at 5 ft/s: Q = 10/448.83 = 0.02228 ft3/s, A = 0.02228/5 = 0.004456 ft2, D = sqrt(4 x 0.004456/pi) x 12 = 0.90 inches. Use 1" nominal pipe.
Practical Tips
- Keep velocity under 8 ft/s to avoid noise and erosion
- Always round up to the next standard pipe size
- PEX has slightly smaller ID than copper at the same nominal size
- For hot water recirculation, use lower velocity (3-4 ft/s)