Fluid Mechanics

Pipe Pressure Drop Calculator

Frictional ΔP and head loss for single-phase flow in a circular pipe (Darcy–Weisbach, Swamee–Jain friction factor).

ΔP = f · (L/D) · (ρ v² / 2), with f from Swamee–Jain or 64/Re

How it works

The tool computes the Reynolds number to pick the flow regime, finds the Darcy friction factor (64/Re for laminar, Swamee–Jain for turbulent), then applies the Darcy–Weisbach equation to get frictional pressure drop and head loss.

Worked example

Water at 2 m/s through 50 m of 100 mm commercial-steel pipe. Turbulent flow, friction factor ≈ 0.018, giving a few kPa of frictional loss.

Inputs

• Inside diameter (mm)
• Pipe length (m)
• Velocity (m/s)
• Density ρ (kg/m³)
• Viscosity μ (Pa·s)
• Roughness ε (mm) — Commercial steel ≈ 0.045 mm

Frequently asked questions

Does this include fittings and valves?

No — it's straight-pipe friction only. Add fitting losses separately using equivalent lengths or K-factors and include them in L or as an added ΔP.

Laminar or turbulent — how is it decided?

By Reynolds number: below ~2300 is laminar (f = 64/Re), above ~4000 is turbulent (Swamee–Jain). Between them is a transitional estimate that should be treated with caution.

What roughness should I use?

Commercial steel is about 0.045 mm; drawn tubing is much smoother (~0.0015 mm) and concrete much rougher. Roughness matters most in fully turbulent flow.

Assumptions

• Circular pipe, full bore, single-phase incompressible flow.
• Fitting/valve losses not included (add equivalent length or K-factors separately).
• Swamee–Jain approximates Colebrook to within ~1% for 5000 < Re < 1e8.

References

• Swamee, P.K. & Jain, A.K. (1976), J. Hydraulics Division, ASCE.
• Darcy–Weisbach and Moody friction methods per Perry's Chemical Engineers' Handbook, Fluid & Particle Dynamics.

Worked examples

• Water at 1.5 m/s in 1-inch schedule 40 pipe
• Water at 1.5 m/s in 2-inch schedule 40 pipe
• Water at 1.5 m/s in 3-inch schedule 40 pipe
• Water at 1.5 m/s in 4-inch schedule 40 pipe
• Water at 1.5 m/s in 6-inch schedule 40 pipe
• Water at 1.5 m/s in 8-inch schedule 40 pipe
• Water at 2 m/s in 1-inch schedule 40 pipe
• Water at 2 m/s in 2-inch schedule 40 pipe
• Water at 2 m/s in 3-inch schedule 40 pipe
• Water at 2 m/s in 4-inch schedule 40 pipe
• Water at 2 m/s in 6-inch schedule 40 pipe
• Water at 2 m/s in 8-inch schedule 40 pipe
• Water at 3 m/s in 1-inch schedule 40 pipe
• Water at 3 m/s in 2-inch schedule 40 pipe
• Water at 3 m/s in 3-inch schedule 40 pipe
• Water at 3 m/s in 4-inch schedule 40 pipe
• Water at 3 m/s in 6-inch schedule 40 pipe
• Water at 3 m/s in 8-inch schedule 40 pipe

Related Fluid Mechanics tools

• Fittings Pressure Drop (K-factors) — Minor losses through elbows, tees, and valves by the ΣK excess-head method — the companion to straight-pipe friction.
• Pipe Velocity & Reynolds — Line velocity, Reynolds number, and flow regime from flow rate and pipe inside diameter.
• Friction Factor (Moody) — Darcy (and Fanning) friction factor from Reynolds number and relative roughness — Colebrook–White solved exactly, with the Swamee–Jain explicit fit for comparison.
• Pipe Line Sizing (velocity) — Required pipe bore for a target line velocity, with the resulting velocity and typical service velocity guidance.

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