Pipe Pressure Drop Calculator

Enter the fluid, pipe diameter, length, flow rate, and material to estimate pressure drop and head loss using the Darcy-Weisbach equation.

Unit System:

Fluid

Pipe inside diameter

Pipe length

Flow rate

GPM

Pipe material

Fittings/valves (total K)

JavaScript is off, so the interactive calculator can't run. See "How it works" below for the Darcy-Weisbach formula and a worked example you can compute by hand.

Pressure drop vs. flow rate

This chart is the central result on this page: it shows how pressure drop rises with flow rate for your current pipe and fluid.

Model-dependent estimate. Results depend on the selected fluid properties, roughness value, and the Darcy-Weisbach model. This is an engineering estimate, not a certified design — verify with a qualified engineer for critical systems.

Advanced options (fluid overrides, pipe-size comparison)

Density override

kg/m³

Viscosity override

Pa·s

Roughness override

Pipe-size comparison

How it works

Flow velocity is the flow rate divided by the pipe's cross-sectional area. The Reynolds number then classifies the flow as laminar or turbulent, and the Darcy friction factor is estimated with the Swamee-Jain approximation (turbulent) or 64/Re (laminar). The Darcy-Weisbach equation gives the pressure drop:

ΔP = f × (L / D) × (ρ × v² / 2)

Where f is the friction factor, L is pipe length, D is inside diameter, ρ is fluid density, and v is velocity. Head loss is ΔP divided by (ρ × g). If you add fittings/valves (K), an additional minor loss of K × ρv²/2 is included in the total pressure drop.

Worked example

10 GPM of 20°C water through 100 ft of 1 in PVC pipe: velocity ≈ 4.0 ft/s, Reynolds number is well into the turbulent range, and the Darcy-Weisbach equation gives a pressure drop on the order of a few psi — see your calculated result above for the exact figure with today's inputs.

Assumptions

Fluid is incompressible and flow is steady and fully developed.
Friction factor uses the Swamee-Jain explicit approximation, not an iterative Colebrook solve.
Minor losses (fittings/valves) are zero unless you enter a total K factor.
Pipe roughness uses representative values per material; actual roughness varies with age and condition.

Reference

Darcy-Weisbach equation and typical roughness values: The Engineering ToolBox — Darcy-Weisbach Equation.

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Frequently asked questions

What causes pressure drop in a pipe?

Friction between the fluid and pipe wall, driven by flow velocity, pipe diameter and length, fluid properties, and pipe roughness.

What equation does this calculator use?

The Darcy-Weisbach equation with the Swamee-Jain explicit friction-factor approximation.

Is the result exact for my system?

No — it depends on your selected fluid properties, roughness, and model assumptions, and excludes minor losses unless entered. Treat it as an estimate, not a certified design.