Duct Size Calculator

How Duct Sizing Works

Duct sizing is a balance between two competing needs: enough cross-section to carry the air without excessive friction loss, and small enough to fit within building cavities and keep material costs down. The fundamental relationship is airflow = velocity × area. If you know the airflow you need (from a Manual J load calc → Manual D duct design), and you pick a target velocity, the required area falls out automatically.

Choosing a Velocity

Lower velocities make quieter, more efficient ducts but cost more in material. Higher velocities are cheaper but noisier. ACCA Manual D recommends: supply trunks 700-900 fpm, supply branches 600-700 fpm, return trunks 500-700 fpm, and return grilles 300-500 fpm. In metric: roughly 3.5-4.5 m/s supply trunk, 3.0-3.5 m/s branch, 2.5-3.5 m/s return. Exceeding 900 fpm in residential systems produces audible whoosh at registers.

Round vs Rectangular

Round ducts have the lowest friction per unit area because their cross-section has no corners to trip turbulence. They are also easier to seal and generally cheaper. Use round ducts wherever you can fit them. Switch to rectangular only when vertical depth is constrained — between 2x10 joists, in shallow soffits, above dropped ceilings. Keep rectangular aspect ratio at 2:1 when possible, 3:1 at most; above that, friction losses climb sharply.

Equivalent Round Diameter

ASHRAE provides a formula to convert a rectangular duct to the round duct diameter with the same friction: De = 1.30 × (a × b)^0.625 / (a + b)^0.25, where a and b are the long and short sides. A 20×10 rectangular duct has an equivalent round of about 15 inches. Use this value when looking up friction loss in round-duct design charts.

CFM to Duct Size Quick Reference

Computed at 700 fpm (typical residential supply branch velocity). These are starting points — drop to 600 fpm for quieter branches, raise to 800-900 fpm for short trunk runs where noise is not a concern. Rectangular dimensions shown are at a 2:1 aspect ratio.

Rough rule of thumb for sizing: 1 sq in of duct area per CFM at 700 fpm. 400 CFM needs ~82 sq in of cross-section, which is a 10" round or 14×6 rectangular. Going to 600 fpm (quieter, longer runs) adds 17% to the area; going to 900 fpm (tight spaces) subtracts 22%.

Frequently Asked Questions

How much CFM does a typical room need?

Rule of thumb: 1 CFM per sq ft for cooling. A 150 sq ft bedroom needs about 150 CFM supply.

Do returns need to be as big as supplies?

Roughly yes — returns typically match or slightly exceed total supply at lower velocities. Starving the return is a common mistake that causes static pressure and furnace overheating.

Can I use flex duct for everything?

Flex duct adds about 20-30% more friction than rigid metal. Use metal for trunks, flex for short branches to registers.

What is static pressure?

Resistance the fan has to overcome. Residential systems design for 0.5 in wc total external static. Oversize ducts or shorten runs if your static is too high.

Should I oversize for future upgrades?

Slightly. Bumping a trunk one size (e.g., 8" round to 10" round) adds 40% area and gives headroom for a future higher-capacity unit.