What is the equal-friction method?

Equal-friction sizing picks a constant friction loss per 100 feet, often 0.08 to 0.10 inches WC, and sizes every duct in the system to that same rate. It is the most common residential and light-commercial method because it self-balances reasonably well and is simple to apply with a ductulator.

How is round diameter calculated?

The tool uses the Darcy friction relation underlying the ASHRAE ductulator: it solves for the diameter that produces the target pressure loss per 100 feet at the given airflow, using standard air density and a typical galvanized-duct roughness. This matches a slide-rule ductulator within normal tolerance.

How does it convert round to rectangular?

It uses the Huebscher equivalent-diameter equation, De = 1.30 × (a×b)^0.625 / (a+b)^0.25, where a and b are the rectangular sides. The tool solves for the unknown side that makes the rectangle's equivalent diameter equal the required round diameter, so both carry the same airflow at the same friction.

What friction rate should I target?

Residential supply trunks commonly use 0.08 to 0.10 inches WC per 100 feet. Lower rates mean larger, quieter ducts; higher rates save space but raise velocity and noise. Keep main-trunk velocity under about 900 feet per minute in homes to avoid noise.

Why does velocity matter?

Velocity drives noise and pressure loss at fittings. Excessive velocity causes whistling registers and high static pressure; too low wastes material and can let air stratify. The tool reports velocity so you can keep it within recommended ranges for the duct's location.

HVAC Duct Sizing Calculator (Equal-Friction Method)

The equal-friction method is the workhorse of residential and light-commercial duct design: pick one friction rate and size every duct to it. This calculator replaces the cardboard ductulator, solving for round diameter directly from your target friction rate and airflow, then converting to an equivalent rectangular size and reporting the resulting velocity.

How it works

The tool solves the Darcy-Weisbach pressure-loss relation that the ASHRAE ductulator is built on, for standard air through galvanized duct:

friction loss per 100 ft  = target (inches WC)
velocity                  = CFM / duct area
diameter                  = solved so loss matches target at the given CFM
rectangular equivalent    De = 1.30 × (a·b)^0.625 / (a+b)^0.25

Given airflow and a target friction rate, it iterates the round diameter until the computed loss per 100 feet matches your target. For rectangular duct it uses the Huebscher equation above and solves the unknown side so the rectangle carries the same airflow at the same friction as the round duct.

Example and tips

A 400 CFM run at a 0.08 inches WC per 100 ft friction rate needs roughly a 9-inch round duct, carrying about 900 feet per minute. If headroom forces a flat duct, an 8-inch fixed side gives a rectangular size near 8 by 10 inches with the same friction. Keep main-trunk velocity under about 900 feet per minute in living spaces to control noise, and remember that fittings add equivalent length the equal-friction rate alone does not capture.