Pipe Friction Loss Calculator
Compute Darcy–Weisbach friction head loss for water flow through a pipe — includes fittings equivalent length.
Inputs
Add up equivalent lengths for elbows, tees, valves (see FAQ).
Results
Formula
h_f = f (L+L_eq)/D · V²/(2g). f from Swamee–Jain (turbulent) or 64/Re (laminar).
Worked example
100 GPM through 200 ft of 3-inch steel pipe (ID = 3 in), 50 ft of fittings, 60°F water: V ≈ 4.5 ft/s, Re ≈ 100,000, f ≈ 0.022, head loss ≈ 6.6 ft (2.9 psi).
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Frequently asked questions
What equivalent length should I use for fittings?
Rules of thumb (in pipe diameters): 90° elbow = 30D, gate valve full open = 8D, globe valve = 340D, tee run-through = 20D, tee branch = 60D. Multiply by pipe diameter (in feet) and sum.
When is flow laminar?
Re < 2000. Rare in commercial water piping unless the pipe is very small or the fluid is viscous (oil, glycol).
Why is Swamee–Jain better than Hazen–Williams?
Hazen–Williams is only valid for water at ambient temperature and specific roughness. Darcy–Weisbach works for any fluid at any temperature.
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