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.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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