Water Flow Rate Calculator
Calculate water flow rate in GPM and LPM from pipe diameter, pressure, and length using Hazen-Williams.
Covers PVC, copper, and steel pipe sizing.
Water flow rate through a pipe is governed by the relationship between pipe diameter, pressure, pipe length, and the friction characteristics of the pipe material.
Hazen-Williams equation (practical form):
Q = 0.2785 × C × d^2.63 × S^0.54
Where:
- Q = flow rate in liters per second (or GPM in imperial)
- C = Hazen-Williams roughness coefficient (material-dependent)
- d = internal pipe diameter in meters (or feet)
- S = hydraulic slope = pressure drop (Pa) / pipe length (m)
Hazen-Williams C coefficients:
| Pipe Material | C Value |
|---|---|
| PVC / Plastic | 150 |
| Copper | 130–140 |
| New steel | 120 |
| Galvanized iron | 100–120 |
| Old cast iron | 80–100 |
| Very corroded pipe | 60–80 |
Unit conversions:
- GPM → L/min: × 3.785
- PSI → kPa: × 6.895
- Inches → mm: × 25.4
Worked example (imperial): 3/4-inch copper pipe (internal diameter ≈ 0.75 in), 50 ft long, pressure drop = 20 PSI. C = 130. S = (20 × 6,895 Pa) / (50 × 0.3048 m) = 137,900 / 15.24 ≈ 9,050 Pa/m. Q ≈ 0.2785 × 130 × (0.01905)^2.63 × (9,050)^0.54 ≈ 0.85 L/s ≈ 13.5 GPM — adequate for a shower (typical demand: 2–3 GPM).
Typical household pressures: 40–80 PSI (275–550 kPa). Water velocity should stay below 3 m/s (10 ft/s) to prevent pipe noise and erosion.