Water Flow Rate Calculator
Calculate water flow rate from pipe diameter and pressure using standard flow equations.
Results
Visualization
How It Works
Flow rate through a pipe depends on pressure, pipe diameter, pipe roughness, and length. The Hazen-Williams equation is the standard method for water distribution systems.
The Formula
Q = ((P x 2.31 x C^1.852 x d^4.87) / (10.67 x L))^(1/1.852) x 448.83
Variables
- Q — Flow rate in GPM
- P — Pressure in PSI
- C — Hazen-Williams roughness coefficient
- d — Inside diameter in feet
- L — Pipe length in feet
Worked Example
A 3/4" copper pipe (C=140) at 50 PSI over 100 feet delivers approximately 5.4 GPM.
Practical Tips
- C-factor decreases as pipes age and corrode
- New copper = 140, old copper = 100-120
- Add 50% to pipe length for fittings as a rule of thumb
- Flow rate increases dramatically with pipe diameter (4.87 power)