Pipe Friction Loss Calculator
Estimate the pressure head lost to friction in any pipe system using the Darcy-Weisbach equation. Use it when designing water supply lines, irrigation networks, or HVAC piping to ensure adequate flow pressure.
Last updated: September 2026
Formula below · 2 sources (asme.org, Wikipedia) · Updated Sep 2026
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About this calculator
The Darcy-Weisbach equation calculates pressure loss due to friction as fluid moves through a pipe: ΔP = f × (L/D) × ρv²/2, where f is the Darcy friction factor, L is pipe length, D is diameter, ρ is density and v is the mean velocity, v = Q / (πD²/4). The calculator finds f from the Reynolds number Re = vD/ν: for laminar flow (Re < 2,300) f = 64/Re, and for turbulent flow it uses the Swamee-Jain equation f = 0.25 / [log₁₀(ε/(3.7D) + 5.74/Re^0.9)]², which matches the Colebrook-White equation within about 1%. Defaults: roughness ε = 0.045 mm (commercial steel; use 0.0015 mm for PVC or copper, 0.26 mm for cast iron) and kinematic viscosity ν = 1.004 × 10⁻⁶ m²/s (water at 20 °C). The result is in pascals; divide by ρg for head loss in metres. Fittings and valves add minor losses not included here. The transitional range (Re 2,300–4,000) is uncertain; the turbulent formula is used there, which is the conservative choice.
How to use
Suppose water (density 1000 kg/m³) flows at 100 L/min through a 50 mm diameter, 30 m long commercial steel pipe. Convert flow rate: 100/60000 = 0.001667 m³/s. Pipe area: π × (0.025)² = 0.001963 m². Velocity: 0.001667/0.001963 = 0.849 m/s. Reynolds number: 0.849 × 0.05 / 1.004 × 10⁻⁶ ≈ 42,270 (turbulent). Friction factor (Swamee-Jain, ε/D = 0.0009): f ≈ 0.0245. Pressure loss: ΔP = 0.0245 × (30/0.05) × 1000 × 0.849²/2 ≈ 5,290 Pa (0.54 m of head). The default 100 m length gives about 17,640 Pa.
Frequently asked questions
What is the Darcy-Weisbach equation and when should I use it?
The Darcy-Weisbach equation is the most accurate and universally applicable formula for calculating pressure loss due to friction in pipes. It works for any fluid, any flow regime (laminar or turbulent), and any pipe material. It is preferred over simpler empirical formulas like Hazen-Williams because it explicitly accounts for fluid density and viscosity through the Reynolds number and friction factor. Engineers use it for water, oil, gas, and chemical process piping design.
How does pipe diameter affect friction head loss in a pipe system?
Pipe diameter has a very strong inverse effect on friction loss — head loss is proportional to 1/D⁴ in the Darcy-Weisbach formula when expressed in terms of volumetric flow rate. This means halving the pipe diameter increases friction loss by a factor of 16 for the same flow rate. Selecting a slightly larger pipe diameter can dramatically reduce energy costs and pump requirements. This trade-off between pipe material cost and long-term pumping energy cost is a core consideration in pipe system design.
What discharge coefficient or friction factor should I use for my pipe material?
The friction factor depends on the pipe’s relative roughness (ε/D) and the Reynolds number. Common roughness values are commercial steel ε ≈ 0.045 mm (the default here), cast iron ε ≈ 0.26 mm, and smooth PVC, copper or drawn tubing ε ≈ 0.0015 mm. For turbulent flow, the calculator uses the Swamee-Jain approximation of the Colebrook-White equation; for laminar flow (Re < 2,300), f = 64/Re regardless of roughness. Enter your pipe’s roughness and the fluid’s kinematic viscosity in the optional fields.
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