How it works
The Darcy–Weisbach equation gives the frictional pressure loss of a straight pipe section. The friction factor λ is 64/Re for laminar flow (Re < 2300); for turbulent flow it is estimated with the Haaland equation, which matches the Colebrook equation to within about 2%.
Formula
v = Q / A Re = v·D / ν λ: 64/Re (laminar), Haaland (turbulent) Δp = λ · (L/D) · ρ·v²/2
Water at 20 °C: ν ≈ 1.0 mm²/s, ρ ≈ 998 kg/m³. New steel pipe has a roughness of about 0.045 mm, plastic pipe 0.0015–0.007 mm. Typical velocities are 1–2 m/s for water lines and 0.5–1 m/s on pump suction. Losses in fittings and valves are not included.
All calculations run in your browser; your inputs never reach us.