What is the Darcy-Weisbach equation?

hf = f × (L/D) × V²/(2g), where hf is friction head loss (m), f is the dimensionless Darcy friction factor, L is pipe length (m), D is internal diameter (m), V is mean velocity (m/s), and g is 9.81 m/s².

How do I find the friction factor?

For laminar flow (Re < 2300), f = 64/Re. For turbulent flow in smooth pipes, this tool uses the Swamee-Jain approximation of the Colebrook-White equation: f = 0.25 / [log₁₀(ε/(3.7D) + 5.74/Re⁰·⁹)]². Enter relative roughness ε/D (ε ≈ 0.000046 m for commercial steel) and the tool computes f automatically, or enter f directly.

What are minor losses?

Minor losses occur at fittings, bends, valves, and entries/exits: hm = K × V²/(2g). Sum the K values for all fittings in the system. Typical values: 90° elbow K ≈ 0.9, fully-open gate valve K ≈ 0.2, pipe entry K ≈ 0.5.

What units does this calculator use?

Inputs are in SI units (meters, m/s). Head loss results are shown in meters (m) and converted to feet (ft). Multiply head in meters by the fluid density and g to convert to pressure (Pa).

Why does pipe diameter have such a strong effect?

Head loss is proportional to L/D and V². Because flow rate Q = V × A = V × π D²/4, keeping Q constant means V ∝ 1/D². So hf ∝ L/D × (1/D)² = L/D⁵ — halving pipe diameter multiplies head loss by 32.

Head Loss Calculator

Name: Head Loss Calculator
Availability: InStock
Author: Nham Vu

Calculate friction head loss in pipes using the Darcy-Weisbach equation. Enter pipe length, diameter, velocity, and friction factor to get head loss in meters and feet.

Pipe Parameters

Pipe Length (m)

Internal Diameter (m)

Flow Velocity (m/s)

Friction Factor

Enter directly Compute from Re & roughness

Darcy Friction Factor (f)

Sum of Minor Loss Coefficients (ΣK) (optional)

Results

Enter pipe parameters and click Calculate

Major Loss (hf)

—

meters

— ft

Minor Loss (hm)

—

meters

— ft

Total Head Loss

—

meters

— ft

Parameter	Value
Friction factor f	—
L/D ratio	—
V²/(2g)	— m
ΣK (minor)	—

Darcy-Weisbach: hf = f × (L/D) × V²/(2g) | hm = ΣK × V²/(2g) | h_total = hf + hm

Summary

Calculate friction head loss in pipes using the Darcy-Weisbach equation. Enter pipe length, diameter, velocity, and friction factor to get head loss in meters and feet.

How it works

Enter pipe length (m) and internal diameter (m).
Enter the average flow velocity (m/s) or use flow rate with pipe area.
Enter the Darcy friction factor — use the known value or let the tool estimate it from relative roughness for turbulent flow.
Optionally enter a sum of minor loss coefficients (K) for fittings, bends, and valves.
Click Calculate to get major head loss (hf), minor head loss (hm), and total head loss in meters and feet.

Use cases

Size pumps to overcome friction losses in long distribution pipelines.
Verify pipe diameter selection meets flow and pressure requirements.
Estimate pressure drop for HVAC chilled-water or hot-water loops.
Check head loss budgets in irrigation system design.
Validate hydraulic model inputs against hand-calculation benchmarks.
Teach the Darcy-Weisbach equation in fluid mechanics coursework.

Frequently Asked Questions

Last updated: 2026-07-23 · Reviewed by Nham Vu