Pressure Drop Calculator (Darcy-Weisbach) — Piping Passion

Units

Inputs

Fluid Preset

Fluid density (ρ)

Dynamic viscosity (μ)

Flow rate (Q)

NPS

Schedule

ID will show below.

Pipe length (L)

Equivalent length: add 30×ID per gate valve, 60×ID per elbow as a quick add.

Pipe roughness (ε)

Result

Pressure drop ΔP

—bar

Velocity

—m/s

Reynolds number & regime

—

Friction factor (Darcy)

—

Formula & Method

ΔP = f × (L / D) × (ρ V² / 2) Turbulent f — Colebrook-White (iterative): 1/√f = −2 · log₁₀[ε/(3.7 D) + 2.51/(Re · √f)] Laminar f = 64 / Re (Re < 2300)

Method: Darcy-Weisbach with Colebrook-White friction factor. Industry reference for clean single-phase flow. Initial guess from Swamee-Jain explicit equation, then iterated to convergence (typically < 6 iterations).

Variables:

f = Darcy friction factor (dimensionless)
L / D = length-to-diameter ratio
ρ = fluid density
V = mean velocity = Q/A
ε = absolute pipe roughness (commercial steel: 0.046 mm)

Limitations: straight-pipe friction loss only. For full system ΔP, add minor losses from fittings, valves, expansions and contractions (use the equivalent-length or K-factor method). Two-phase flow needs Lockhart-Martinelli or Beggs-Brill, not this calculator.

Brownfield tip: measured pressure drop on an old line is often 2–4 times the calculated value because internal roughness has increased from corrosion, scale and wax. If the line is > 15 years old in dirty service, double your ε from new-pipe value.

⚠ For preliminary sizing only Results are based on nominal ASME dimensions and typical material densities. They do not account for manufacturing tolerances, coatings, supports, flanges, fittings, corrosion allowance, or actual site conditions. All final designs must be verified by a qualified engineer and validated against the applicable code edition. Piping Passion accepts no liability for decisions made using this tool.