Pipe Flow Calculator
Head loss, velocity, and Reynolds number for water in pipe. Darcy-Weisbach (Colebrook) or Hazen-Williams.
Formulas+
Darcy-Weisbach: h_f = f · (L/D) · V²/(2g) — Colebrook for f
Hazen-Williams (US): h_f = 4.52 · Q¹·⁸⁵² · L / (C¹·⁸⁵² · d⁴·⁸⁷)
Reynolds Re = VD/ν, with ν ≈ 1.08 × 10⁻⁵ ft²/s for water at 60 °F.
Straight-pipe friction only. Add fittings, elevation, and pump curves before specifying a system. Final piping design should be sealed by a licensed Mechanical or Civil P.E.
Frequently asked questions
Darcy-Weisbach vs. Hazen-Williams — which do I use?+
Hazen-Williams: water-only, turbulent, room temperature, simple — fine for fire mains and domestic water. Darcy-Weisbach: any fluid, any regime, more accurate; required for compressible flow, slurries, or hot/cold water far from 60 °F.
How is the friction factor solved?+
Colebrook-White equation, iterated until convergence (typically 6-8 iterations). Roughness ε defaults are 0.0018 in for PVC, 0.006 in for steel, 0.01 in for galvanized.
What if Reynolds is in the transition zone?+
Below Re ~2,300 flow is laminar (f = 64/Re). Above Re ~4,000 it's turbulent (Colebrook). The 2,300-4,000 transition zone is unpredictable — design out of it.
Why is my pipe velocity so high?+
Domestic water targets 5-8 ft/s; >10 ft/s causes erosion and water hammer. Up-size the pipe or split the run.
Does this include fittings and valves?+
No — straight-pipe friction only. Add equivalent length per Crane TP-410 or 2K method for elbows, tees, valves, then re-run.
Can I model gas or steam?+
No. This tool is for incompressible liquids (water and similar). Gas needs Weymouth/Panhandle; steam needs two-phase flow analysis.