Water Pressure Loss Calculator
Calculate friction pressure loss in a water pipe from flow rate, diameter, length and material using the Hazen-Williams equation.
How it works
h (ft per 100 ft) = 0.2083 · (100 ÷ C)^1.852 · Q^1.852 ÷ d^4.8655 · psi = h × 0.4331
Friction turns some of your pressure into heat as water rubs against the pipe wall, and the loss climbs steeply — almost with the square of flow rate. The Hazen-Williams equation is the plumbing-standard estimate for water, using the C factor to describe how smooth the pipe is. Undersized pipe is the usual culprit behind weak showers: halving the diameter roughly multiplies the pressure loss by 28 at the same flow.
Worked example
Push 10 GPM through 50 ft of 3/4 in copper (C = 140). Hazen-Williams gives about 32 ft of head loss per 100 ft, so 50 ft loses 16 ft of head — around 7 psi — while the water races along at over 7 ft/s, a sign the pipe is too small for that flow.
Sources: Hazen-Williams equation
Frequently asked questions
How much pressure loss is acceptable?
Plumbers commonly design supply runs for no more than about 3–5 psi loss between the meter and the fixture, and keep velocity under 8 ft/s. Larger losses mean thin flow at the tap, especially upstairs.
Why is flow raised to the 1.85 power?
Friction in turbulent pipe flow rises faster than linearly with speed. The 1.852 exponent is the empirically fitted value in Hazen-Williams, so doubling the flow nearly quadruples the pressure loss.
Does temperature matter?
Hazen-Williams assumes water around room temperature. It is accurate for typical cold and warm plumbing; very hot water is slightly less viscous, giving marginally lower loss, which this estimate ignores.
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This calculator is for educational purposes. Double-check important results before acting on them.