Darcy-Weisbach Pressure Drop Calculator
Pipe friction pressure drop and head loss with the Darcy-Weisbach equation and the Colebrook friction factor — any fluid, pipe material and size, with Reynolds number, velocity and flow regime.
How it works
h_f = f × (L ÷ D) × v² ÷ 2g · Δp = ρ·g·(h_f + ΣK·v² ÷ 2g) · v = Q ÷ (π·D² ÷ 4) · Re = v·D ÷ ν · laminar (Re < 2,300): f = 64 ÷ Re · turbulent: Colebrook 1/√f = −2·log₁₀(ε/(3.7·D) + 2.51/(Re·√f))
The Darcy-Weisbach equation is the general, dimensionally consistent way to compute friction losses in full pipes for any liquid — unlike Hazen-Williams, which only suits water near room temperature. The friction factor comes from the Colebrook-White equation (the formula behind the Moody chart), solved here by iteration, and depends on the Reynolds number and the relative roughness of the pipe wall. Fittings and valves are added as minor-loss coefficients K times the velocity head.
Worked example
Water at 20 °C flowing at 20 m³/h through 100 m of 80 mm commercial steel pipe moves at 1.11 m/s, a Reynolds number of 88,067 (turbulent). With ε/D = 0.00056 the Colebrook friction factor is 0.0209, so the head loss is 0.0209 × (100 ÷ 0.08) × 1.11² ÷ (2 × 9.81) = 1.63 m and the pressure drop 15.94 kPa (about 0.16 bar).
Sources: Engineering ToolBox — Darcy-Weisbach pressure and major head loss · Wikipedia — Darcy–Weisbach equation
Frequently asked questions
What is the difference between Darcy-Weisbach and Hazen-Williams?
Darcy-Weisbach works for any Newtonian fluid, temperature and flow regime because the friction factor depends on Reynolds number and roughness. Hazen-Williams is an empirical shortcut valid only for water at ordinary temperatures in turbulent flow.
Which friction factor does this calculator use?
The Darcy (Moody) friction factor, four times the Fanning factor. Laminar flow uses 64 ÷ Re; turbulent flow solves the Colebrook-White equation, starting from the Swamee-Jain approximation.
What velocity should I design water pipes for?
Typical guidance is 1–2 m/s (3–7 ft/s) for water distribution and pump discharge, and under about 1 m/s on pump suction lines, to keep losses, noise and water hammer reasonable.
How do I include elbows and valves?
Add up their K values and enter the total; each contributes K × v² ÷ 2g of head. Alternatively add their equivalent pipe length to the straight length.
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This calculator is for educational purposes. Double-check important results before acting on them.