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Pipe Friction Loss Calculator

Reviewed by the CalculatorHub team, edited by James Graham. Darcy-Weisbach equation with Swamee-Jain friction factor. Last verified 14 June 2026.

The Darcy-Weisbach equation is the internationally recognised method for calculating friction head loss in pipes. This calculator uses the Swamee-Jain explicit approximation to the Colebrook-White equation for turbulent friction factor, and computes the Reynolds number to determine flow regime. Enter your pipe geometry, flow conditions, and material roughness to get head loss and pressure drop.

Total straight pipe length (exclude fittings or add as equivalent length)
Pipe internal bore diameter in millimetres
Mean flow velocity in the pipe. Typical range: 0.5 to 3 m/s for water.
Absolute roughness of pipe interior surface
Water at 20 C: 1.004 mm^2/s. Water at 60 C: 0.474 mm^2/s. Air at 20 C: 15.1 mm^2/s.
Water at 20 C: 998 kg/m3. Air at 20 C: 1.204 kg/m3.
-- (turbulent)
--
-- m
-- kPa

Darcy-Weisbach formula

hf = f × (L/D) × v2 / (2g)
Re = v × D / nu
f (laminar, Re < 2,300) = 64 / Re
f (turbulent) = 0.25 / [log10(e/(3.7D) + 5.74/Re0.9)]2
dP = rho × g × hf

Where: f = Darcy friction factor, L = pipe length (m), D = diameter (m), v = velocity (m/s), g = 9.81 m/s^2, Re = Reynolds number, nu = kinematic viscosity (m^2/s), e = pipe roughness (m), rho = density (kg/m^3). The Swamee-Jain approximation has an accuracy within 3% of the Colebrook-White equation for Re between 5,000 and 10^8 and e/D between 0.000001 and 0.05.

Recommended pipe velocities

  • Cold water supply: 0.5 to 1.5 m/s (ASHRAE Handbook).
  • Hot water heating: 0.5 to 1.5 m/s.
  • Industrial process water: 1.5 to 3.0 m/s.
  • Low-pressure steam: 15 to 30 m/s.
  • Higher velocities increase friction loss significantly (quadratic relationship with velocity).

Pipe friction loss calculator: frequently asked questions

What is the Darcy-Weisbach equation?

The Darcy-Weisbach equation calculates head loss due to friction in a pipe: hf = f * (L/D) * (v^2 / 2g), where f is the Darcy friction factor (dimensionless), L is pipe length (m), D is internal diameter (m), v is mean flow velocity (m/s), and g = 9.81 m/s^2. This is the standard formula per ASME and ISO for pipe friction loss.

How do I find the Darcy friction factor?

For laminar flow (Re less than 2,300), f = 64/Re. For turbulent flow (Re greater than 4,000), use the Colebrook-White equation or Moody chart. The Swamee-Jain explicit approximation is: f = 0.25 / [log10(epsilon/(3.7*D) + 5.74/Re^0.9)]^2, where epsilon is pipe roughness (m). For commercial steel, epsilon = 0.046 mm; for PVC, epsilon = 0.0015 mm.

What is the Reynolds number?

The Reynolds number Re = rho * v * D / mu = v * D / nu, where rho is density (kg/m^3), v is velocity (m/s), D is diameter (m), mu is dynamic viscosity (Pa.s), and nu is kinematic viscosity (m^2/s). For water at 20 C, nu = 1.004 x 10^-6 m^2/s. Re less than 2,300 is laminar; Re greater than 4,000 is turbulent.

How do I account for fittings and valves?

Minor losses from fittings are added either as equivalent lengths of pipe (Le = K * D / f) or directly as loss coefficients K: h_minor = K * v^2 / (2g). ASHRAE Fundamentals and CIBSE Guide C provide K factor tables. Add all equivalent lengths to the pipe length L before applying the Darcy-Weisbach equation.

What is the difference between head loss and pressure drop?

Head loss hf is expressed in metres of fluid column. Pressure drop dP = rho * g * hf in Pascals. For water at 20 C, 1 m of head loss = 9,789 Pa = 9.79 kPa. For pressure loss in different fluids, multiply head loss by the fluid density and g.

Official sources

Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.