What equation does this use?

It uses the Darcy-Weisbach equation for pressure drop with the friction factor solved from the Colebrook-White relation, iterated to convergence. The absolute roughness is set for galvanized sheet metal, about 0.0003 feet.

What friction rate should I design for?

Residential systems are commonly sized for about 0.08 to 0.10 inches of water column per 100 feet. Keeping every run near that target gives balanced airflow and reasonable blower static pressure across the system.

Why does duct velocity matter?

Velocity that is too high causes noise and excessive pressure loss, while velocity that is too low can let dirt settle. Residential supply trunks typically run 600 to 900 FPM and branches lower, while commercial duct can run higher.

Does this include fitting losses?

No. It gives friction for straight round duct only. Add the equivalent length of elbows, tees, and transitions from a fitting-loss table, then sum the longest path to find total static pressure for blower selection.

Does it work for rectangular duct?

Directly it sizes round duct. For rectangular duct, first convert to an equivalent round diameter using a standard equal-friction chart, then enter that diameter and your airflow here.

HVAC Duct Friction Loss Calculator

Friction loss is the heart of duct sizing: every foot of duct steals a little static pressure from your blower. This calculator solves the real fluid-dynamics equations to give friction loss per 100 feet of round galvanized duct, and warns you when a run is moving air too fast.

How it works

The tool computes velocity from airflow and area, finds the Reynolds number, and solves the Colebrook-White equation for the friction factor before applying Darcy-Weisbach:

velocity (FPM) = CFM / duct_area
Re             = velocity_fps × diameter_ft / kinematic_viscosity
1/√f           = -2 log10( ε / (3.7 D) + 2.51 / (Re √f) )
ΔP per 100 ft  = f × (100 / D) × (ρ × V² / 2),  converted to in. w.c.

This is the same physics behind the equal-friction duct charts, computed directly rather than read off a curve.

Example and tips

400 CFM through 8 inch round duct runs about 1150 FPM with roughly 0.13 in. w.c. of loss per 100 feet — a bit fast and lossy for residential, so bumping to 9 or 10 inch duct brings velocity and friction back into the comfortable range. Size every run to a consistent friction rate, add equivalent lengths for fittings, and total the worst-case path to pick a blower with enough external static pressure.