Ductulator: round duct size, from CFM and friction rate.

Short answer

Round duct size from CFM and friction rate uses the Darcy equation with the Colebrook friction factor. At 400 CFM and 0.10 in. w.g. per 100 ft the calculated diameter is 9.68 in, so use 10 in round duct, which moves the air at 733 ft/min with a friction rate of 0.085.

Worked example

A trunk must carry 400 CFM and the design friction rate is 0.10 in. w.g. per 100 ft. The model gives a calculated diameter of 9.68 in, which rounds up to 10 in. In the 10 in duct the air moves at 733 ft/min and loses 0.085 in. w.g. per 100 ft, a little less than the target because the duct is slightly bigger than calculated.

Formula, assumptions and sources

Friction loss comes from the Darcy equation, loss = f x (L / D) x (density x velocity squared / 2), converted to inches of water, with the Colebrook equation giving the friction factor f from the Reynolds number and the duct roughness. ASHRAE's Duct Design chapter publishes its round-duct friction chart for standard air (0.075 lbm/ft3) and a galvanized-steel absolute roughness of 0.0003 ft, and this tool uses the same two values; the dynamic viscosity of air (about 0.0000122 lbm/ft-s near 70 F) is a physical property. The diameter is found by solving for the size that gives your friction rate, then rounded up to a whole inch. The DOE Building America guideline defines friction rate as the pressure loss between two points a set length apart, in inches of water column per 100 ft (iwc/100 ft). Flex duct, rougher materials, fittings, duct length, high altitude and non-standard air temperature all change the real pressure loss and are not modelled. Very slow air (Reynolds number under 2,300) is outside the turbulent model and returns no result.

Source: ASHRAE Handbook - Fundamentals (2017), Chapter 21: Duct Design (retrieved 2026-10-15)

Source: U.S. DOE Building America, Advanced Strategy Guideline: Air Distribution Basics and Duct Design (IBACOS, December 2011) (retrieved 2026-10-15)

Safety and limits

This is a planning aid, not professional advice. Duct, furnace and air-conditioner design affects comfort, combustion safety and code compliance; a licensed HVAC contractor should perform a load calculation, size the system and verify airflow on site.

FAQ

What is a ductulator?

A ductulator is a duct slide rule or calculator that links airflow in CFM, round duct diameter, air velocity and friction rate in inches of water per 100 ft. Give it two of those and it returns the others.

What friction rate should I use?

Use the rate from your static pressure budget: available static pressure times 100 divided by the total effective length of the longest run. The Duct Friction Rate Calculator on this site does that arithmetic.

Why does the tool round the diameter up?

Round duct is sold in standard sizes, so you pick the next size up. A larger duct lowers friction and air speed below your target, which is the safe direction for noise and pressure.

Does this work for rectangular or flex duct?

It sizes round metal duct. For a rectangular duct, convert the round size with the Round to Rectangular Duct Calculator. Flex duct has more friction than the galvanized steel assumed here, so ask the manufacturer for its friction data.