Darcy-Weisbach friction loss calculator
Darcy-Weisbach friction loss sketches show up when a hydraulics course, manufacturer note, or civil/fire comparison uses head loss instead of (or alongside) Hazen-Williams. The teaching form is hf = f × (L/D) × (V2 / (2g)), with head loss hf in feet of fluid, friction factor f (dimensionless), length L and diameter D in consistent length units, mean velocity V, and g ≈ 32.174 ft/s2 for water in U.S. customary units. Pressure loss in psi is roughly hf × 0.433 for freshwater. This page explains that sketch and gives a free browser calculator. It is for learning and quick checks—not a substitute for the criteria adopted by your authority having jurisdiction, manufacturer pipe data, a sealed hydraulic calculation, or review by a licensed design professional.
How Darcy-Weisbach friction loss is used
- Enter a Darcy friction factor f from your Moody / Colebrook method, manufacturer data, or a textbook table for the Reynolds number and relative roughness you are studying.
- Enter pipe length L (ft) and internal diameter D (in); the calculator converts D to feet for the L/D term.
- Enter mean velocity V (ft/s) directly, or enter flow Q (gpm) and let the page compute V = Q / (2.448 × d_in2) with d in inches (common U.S. fire-protection teaching shortcut).
- Compute hf from the formula above, then approximate psi ≈ hf × 0.4333 for freshwater at ordinary temperatures.
- Compare with a Hazen-Williams sketch on the same segment when your project method uses HW instead—do not mix methods inside one sealed calculation without knowing which edition and method apply.
Free calculator
Educational approximation only. Formula: hf = f (L/D) (V2/(2g)) with g = 32.174 ft/s2; psi ≈ hf × 0.4333. Nothing you enter leaves your browser. For Hazen-Williams psi/ft see the friction loss calculator or the fuller Hazen-Williams calculator. For flow regime checks see the Reynolds number calculator. Multi-segment HW paths live in the Sprinkler Pipe Friction Loss workbook.
Worked example
A segment with f = 0.02, L = 100 ft, D = 2.067 in (D = 0.1723 ft), and V = 10 ft/s: hf = 0.02 × (100/0.1723) × (100/(2×32.174)) ≈ 18.0 ft of head, or about 7.8 psi freshwater. Always confirm the friction factor method, published internal diameter, and check the result against the adopted edition, manufacturer data, and AHJ—not this page alone.
Next steps: check flow regime with the free Reynolds number calculator, compare a Hazen-Williams sketch with the friction loss calculator, convert pressure and head with PSI to feet of head, and build multi-segment HW paths in the Sprinkler Pipe Friction Loss workbook ($29).