Pressure Drop Calculator
Find pressure loss using the Darcy-Weisbach equation.
Calculate
Formulas
ΔP = f(L/D)(ρv²/2)Laminar: f = 64/Re. Turbulent: use Moody chart.
Darcy-Weisbach
Most general pressure drop formula. Smooth pipes: f = 0.01-0.03. Rough pipes: 0.03-0.05. Add fitting losses using equivalent length or K-factors.
How Pressure Drop Is Calculated
As fluid flows through a pipe, friction with the walls causes a loss of pressure along its length. The Darcy-Weisbach equation gives this drop:
ΔP = f(L/D)(ρv²/2)ΔP is the pressure drop, f is the Darcy friction factor, L is pipe length, D is diameter, ρ is fluid density, and v is flow velocity.
Pressure drop rises sharply with velocity (squared) and length, and falls with larger diameter. This drives pump sizing and pipe selection.
Worked Example
Water (ρ = 1000) at 2 m/s in a 0.1 m pipe, 50 m long, f = 0.02:
ΔP = 0.02 × (50/0.1) × (1000 × 2² / 2)
ΔP = 0.02 × 500 × 2000 = 20000 Pa = 0.2 bar
What Affects Pressure Drop
| Factor | Effect on ΔP |
|---|---|
| Double the velocity | 4× increase |
| Double the length | 2× increase |
| Double the diameter | ~32× decrease |
Diameter has the strongest effect: increasing pipe size dramatically reduces pumping costs over long runs.
Frequently Asked Questions
What is the friction factor f?
A dimensionless number depending on the Reynolds number and pipe roughness. It is read from a Moody chart or computed from correlations like Colebrook.
Why does diameter matter so much?
Larger diameter both lowers velocity (for the same flow) and increases D in the denominator, compounding into a steep reduction in pressure drop.
How does this relate to pump selection?
The pump must supply enough head to overcome the total pressure drop plus any elevation change. Underestimating ΔP leads to insufficient flow.