Pressure Drop Calculator

What is Pressure Drop in Pipes?

When fluid flows through a pipe, friction between fluid and pipe walls causes a pressure drop (head loss). This pressure must be supplied by the pump, gravity head, or supply pressure. Accurate pressure drop calculation is essential for pump sizing and system performance.

The Darcy-Weisbach Equation

ΔP = f × (L/D) × (ρv²/2)

Or in head loss form: h_f = f × (L/D) × (v²/2g)

Where:

• f = Darcy friction factor (from Moody chart; use Colebrook equation for turbulent flow)
• L = pipe length (m)
• D = internal diameter (m)
• ρ = fluid density (kg/m³)
• v = fluid velocity (m/s)

For laminar flow: f = 64/Re. For turbulent flow, use the Swamee-Jain approximation: f = 0.25 / [log(ε/3.7D + 5.74/Re⁰·⁹)]²

Practical Examples

Example: Steel pipe (ε = 0.046mm), 50mm diameter, 50m long, water at 2 m/s. Re = 100,000 (turbulent). f ≈ 0.021. ΔP = 0.021 × (50/0.05) × (1000 × 4 / 2) = 0.021 × 1000 × 2000 = 42,000 Pa = 42 kPa = 4.3 m head.

Fitting Losses

Add equivalent pipe lengths for fittings (elbows, valves, tees). A ball valve fully open ≈ 8D; a 90° elbow ≈ 30D; gate valve fully open ≈ 13D.