What equation does this calculator use?

It uses the Darcy-Weisbach equation: h_f = f × (L/D) × (v²/2g), where f is the Darcy friction factor, L is pipe length, D is inner diameter, v is flow velocity, and g is gravitational acceleration.

How is the friction factor determined?

For laminar flow (Re < 2300), f = 64/Re. For turbulent flow (Re ≥ 4000), the Swamee-Jain explicit approximation of the Colebrook equation is used. The transitional zone (2300–4000) shows a warning since flow behavior is unpredictable.

What is absolute roughness?

Absolute roughness (ε) is the average height of surface irregularities inside the pipe, in millimeters. Smooth plastic pipes have very low roughness (~0.0015 mm), while older cast iron pipes can have values above 0.25 mm.

Does this include minor losses (fittings, valves)?

No. This calculator covers only major (friction) losses along a straight pipe. For a complete system, add minor losses using K-factor or equivalent-length methods separately.

What units are used for flow rate?

Flow rate can be entered in liters per second (L/s), liters per minute (L/min), cubic meters per hour (m³/h), or US gallons per minute (GPM). The tool converts internally to SI for calculation.

Why does the result change with temperature?

Water viscosity drops significantly with temperature, which raises the Reynolds number and can shift the flow regime, producing a different friction factor and therefore a different pressure loss even at the same flow rate.

Water Pressure Loss Calculator

Name: Water Pressure Loss Calculator
Availability: InStock
Author: Nham Vu

Compute friction head loss and pressure drop in a pipe using the Darcy-Weisbach equation with automatic friction factor estimation.

Pipe Parameters

Pipe Length

Inner Diameter

Flow Rate

Pipe Material

Fluid / Temperature

Results

Enter values and click Calculate

Darcy-Weisbach Reference

h_f = f × (L / D) × (v² / 2g)

f = friction factor (Swamee-Jain for turbulent)

Re = ρ v D / μ | v = Q / A

ΔP = ρ g h_f

Summary

Compute friction head loss and pressure drop in a pipe using the Darcy-Weisbach equation with automatic friction factor estimation.

How it works

Enter the pipe length, inner diameter, and flow rate.
Select the pipe material or enter a custom roughness value.
Choose the fluid (water at various temperatures, or set custom density/viscosity).
The tool calculates the Reynolds number and friction factor via the Swamee-Jain approximation.
Head loss (m) and pressure drop (kPa and psi) are displayed instantly.

Use cases

Size pumps for building water supply systems.
Verify pipe diameters during HVAC and plumbing design.
Estimate pressure loss in irrigation and fire-suppression networks.
Check whether existing pipes can deliver required flow rates.
Compare different pipe materials and diameters for an upgrade project.
Validate hydraulic models in civil and mechanical engineering courses.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu