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, the form this calculator uses (US customary):
h = 0.2083 × (100/C)^1.852 × Q^1.852 ÷ d^4.8655
Where:
- h = head loss in feet of water per 100 feet of pipe
- Q = flow rate in gallons per minute (GPM)
- C = Hazen-Williams roughness coefficient (material-dependent)
- d = internal pipe diameter in inches
Turned around to give flow from pressure, which is what you actually want:
Q = [ h × d^4.8655 ÷ (0.2083 × (100/C)^1.852) ]^(1/1.852)
A warning about the metric version. You will also see Q = 0.2785 × C × d^2.63 × S^0.54, with Q in cubic metres per second and d in metres. That one is correct too, but S is a dimensionless slope, metres of head lost per metre of pipe. It is not pressure in pascals divided by length. Feeding pascals into S inflates the answer by a factor of several hundred, and it is the single commonest mistake people make with this equation. Convert pressure to head first: 1 psi lifts water 2.31 feet, and 1 kPa lifts it 0.102 metres.
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, and you are willing to spend 5 PSI of pressure on friction. Copper is C = 135.
Head loss per 100 ft: h = 5 × 2.31 × 100 ÷ 50 = 23.1 ft/100ft
Q = [23.1 × 0.75^4.8655 ÷ (0.2083 × (100/135)^1.852)]^(1/1.852) = 8.06 GPM (30.5 LPM)
At that flow the water moves 5.9 ft/s, inside the safe band, and 8 GPM covers a shower and a basin running together. Ask the same pipe for 20 PSI of drop instead and it delivers 17.0 GPM at 12.4 ft/s, which is past the point where copper starts to erode at the elbows. The pipe is not the limit there; the velocity is.
Typical household pressures: 40–80 PSI (275–550 kPa). Note that this is the static pressure at the main, and only part of it is available to spend on friction in any one run.
Velocity limits, which usually bind before pressure does. Keep copper below about 8 ft/s (2.4 m/s). Above that, water moving past the inside of an elbow strips the protective oxide film faster than it re-forms, and the fitting thins from the inside out. This is erosion-corrosion, and it is the reason a copper system can spring a pinhole leak at a fitting after fifteen years while the straight runs are fine. Plastic tolerates more, up to about 10 ft/s, because there is no oxide film to lose. Both materials get noisy well before they get damaged.
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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