Pipe Pressure Drop Calculator
Enter flow, size, length and material roughness to get head loss and pressure drop — the numbers that decide whether the far shower gets decent pressure.
Last updated: 2026-09-28
How the calculation works
- Reynolds number from velocity and internal diameter picks the regime: laminar below 2300, turbulent above.
- Turbulent friction comes from the Swamee-Jain explicit equation (≈1% vs Colebrook); laminar is the exact 64/Re.
- Head loss converts to pressure through ρ·g — about 9.8 kPa per metre of water.
Formula
h = f · (L/D) · V²/2g Δp = ρ·g·h f: Swamee-Jain (turbulent), 64/Re (laminar)
| Symbol | Meaning | Unit |
|---|---|---|
h | Head loss | m |
f | Darcy friction factor | — |
Re | Reynolds number | — |
Worked example
Interpreting the result
Pressure loss is the budget every distribution system spends: the available pressure at the street minus everything the path consumes must still satisfy the worst fixture (typically 100 kPa / 15 psi for showers). Loss scales with V² and L/D — velocity is the lever, which is why upsizing one trade size transforms long runs. Old steel pipe's rising roughness quietly strangles flow: a 1 mm roughness multiplies friction several-fold versus new copper.
Assumptions
- Water at 20 °C, straight pipe, full bore.
- No elevation change or fitting losses included.
- Smooth-wall Darcy-Weisbach model: published copper tables (Hazen-Williams, C = 145) read ~10% higher for small tubes at domestic flows.
Limitations
- Fittings, valves and bends dominate short runs — add equivalent length or 20–50% margin.
- For other fluids, adjust viscosity and density (this model is water-specific).
Frequently asked questions
How much pressure do I lose per metre of pipe?
It depends on velocity: at 1.5 m/s in 20 mm copper, roughly 1.5 kPa per metre. Loss scales with V², so 3 m/s costs 4× as much.
What is the Swamee-Jain equation?
An explicit approximation of the Colebrook friction factor: f = 0.25/[log₁₀(ε/3.7D + 5.74/Re⁰·⁹)]². Accurate to about 1% for turbulent water mains without iterating.