Darcy-Weisbach Equation Calculator

When to use Darcy-Weisbach

The Darcy-Weisbach equation is the dimensionally correct, fluid-and-temperature-aware way to compute pipe friction loss. It applies to any Newtonian fluid in any flow regime, in pipes of any roughness. For water-supply work in the typical pressure-and-temperature range, Hazen-Williams is the simpler shortcut — but Hazen-Williams is calibrated for water at common conditions and falls apart for hot water, low-Reynolds flow, or non-water fluids.

Pipe roughness ε values

• Drawn tubing (copper, brass): 0.000059 in (0.0015 mm)
• PVC, smooth plastic: 0.0006 in (0.0015 mm)
• Commercial steel: 0.0018 in (0.045 mm)
• Galvanized iron: 0.006 in (0.15 mm)
• Cast iron: 0.0102 in (0.26 mm)
• Concrete pipe: 0.012 to 0.12 in (0.3 to 3.0 mm)
• Riveted steel: 0.036 to 0.36 in (0.9 to 9.0 mm)

Kinematic viscosity for water

Water at 60°F (15.6°C) has ν ≈ 1.08 × 10⁻⁵ ft²/s (1.13 × 10⁻⁶ m²/s). At 40°F use 1.66 × 10⁻⁵ ft²/s; at 80°F use 0.93 × 10⁻⁵ ft²/s. Viscosity changes ~50% between 40°F and 100°F, which matters for laminar-flow calculations but is usually negligible for fully turbulent water-distribution work.

Reynolds number regimes

• Re < 2,300: laminar — friction factor is f = 64/Re, independent of roughness.
• 2,300 ≤ Re ≤ 4,000: transitional — friction factor uncertain.
• Re > 4,000: turbulent — Swamee-Jain (or Colebrook) applies.

Reference: Crane Technical Paper No. 410, Flow of Fluids Through Valves, Fittings, and Pipe. Swamee, P.K., Jain, A.K. (1976). "Explicit equations for pipe-flow problems." J. Hydraulic Div., ASCE, 102 (5), 657–664.