Best Engineer Tools
enptes
Hydraulic

Head Loss Calculator (Darcy-Weisbach)

Calculate friction head loss along a pipe using the Darcy-Weisbach equation, with the friction factor estimated automatically from flow conditions and pipe roughness - no Moody chart lookup required.

Inputs

Result

Enter values above to calculate

These calculators provide estimates and educational results. Always verify critical engineering calculations against applicable standards and qualified professional review.

Related Calculators

What Is It

Head loss is the energy a fluid loses to friction as it flows through a pipe, expressed as an equivalent height of fluid column. It's the dominant factor in sizing pumps correctly and predicting pressure drop across a piping system.

How to Calculate

Enter the flow rate, pipe internal diameter, pipe length and material (which sets its roughness). The calculator computes velocity and Reynolds number, estimates the Darcy friction factor using the Colebrook-equivalent Swamee-Jain formula (or the laminar 64/Re formula for slow flows), and applies the Darcy-Weisbach equation to get head loss.

Example

100 L/min through 50m of 25mm PVC pipe: velocity about 3.4 m/s, Reynolds number in the turbulent range, friction factor around 0.03 for smooth PVC, giving a head loss on the order of several meters - enough to matter significantly in pump sizing for this small a pipe.

Recommendations

Head loss increases roughly with the square of velocity and is highly sensitive to pipe diameter - before accepting a high calculated head loss, check whether a modest increase in pipe diameter would be more economical than the extra pump power required to overcome it.

Frequently Asked Questions

Why does the calculator need pipe material?

Pipe roughness (which varies significantly by material and age) directly affects the friction factor for turbulent flow - a rougher pipe like galvanized steel or aged cast iron produces substantially more head loss than a smooth pipe like PVC or copper at the same flow rate.

What is the Swamee-Jain equation?

It's a widely used explicit approximation of the implicit Colebrook equation for the Darcy friction factor in turbulent flow, avoiding the need for iterative solving or a Moody chart while remaining accurate within a few percent for most engineering purposes.

Does this account for fittings and valves?

No - this calculates only straight-pipe friction (major losses). Fittings, valves and bends add additional 'minor losses' that must be estimated separately and added to this result for a complete system head calculation.

Last reviewed: 2026-09-12