Ductwork Size & Cost Calculator

Start from total airflow, not floor area alone: a common planning rule is about 0.9 CFM per square foot at an average climate load, which is the shorthand behind the more familiar 400 CFM per ton of AC or furnace capacity. Split that total evenly across your planned registers to get a per-branch CFM, then match trunk and branch diameter to whatever size duct can carry that airflow at a reasonable velocity — too small and the system is loud and starved, too large and it wastes sheet metal and airflow control. This calculator runs that whole chain and rounds to real stock sizes.

At the 0.10 in.wg per 100 ft friction rate residential systems are designed to, a 6 in smooth metal branch carries roughly 110 CFM, an 8 in about 225 CFM and a 12 in about 700 CFM. Insulated flex of the same diameter carries roughly 70% of those figures — around 80, 180 and 560 CFM — because the corrugated liner drags on the airflow. Trunk duct is sized the same way but as a rectangle at a fixed 8 in height, so a wider trunk moves more air down the same duct depth. Undersize either and velocity climbs past the point where you hear it; a whistling register is almost always a branch too small for the air being pushed through it.

Insulated flex duct is faster to install, comes pre-insulated so it skips the wrap step, and bends around framing without extra fittings, which is why it dominates residential branch runs. It does have more internal friction per foot than smooth rigid pipe, and it sags and loses capacity if it isn't pulled tight and strapped every few feet. Rigid round duct moves more air for the same diameter and holds up better long-term, but it costs more in labor to cut, crimp and fabricate on site. Most homes mix a rigid trunk with flex branches, which is this calculator's default.

Plan one supply register per room at minimum, and split a large room into two branches rather than pushing a single run past roughly 150–200 CFM, which is where a residential register starts to get audible. That threshold is the real reason register count matters: eight registers in a 2,000 sq ft house forces every branch up to a size that whistles, while twelve keeps each one comfortably in the 6–8 in range. Returns are fewer and larger — one central return per floor is common in smaller homes, while larger or multi-story homes add a return per major zone so air can actually circulate back to the air handler instead of pressurizing closed-door rooms.

Running flex branch duct, hanging trunk sections and setting registers are all reachable DIY tasks with basic sheet metal tools, and this is a common owner-builder project in unfinished basements and attics. Where it gets harder is airtight sealing — mastic on every rigid joint, foil tape on every flex connection — since a system that leaks 20–30% of its air at the joints performs badly no matter how well it was sized. Tying into an existing plenum or air handler, and any gas furnace work nearby, is worth having a licensed HVAC contractor review even on an otherwise DIY job.

The trunk is the single large duct that carries the full system's airflow away from the air handler or furnace, usually a rectangular rigid run through the attic, basement or a soffit. Branch ducts tap off the trunk through a take-off collar, one per register, each sized to carry only that room's share of the total airflow. Return duct works the same way in reverse — branch grilles pull room air back into a return trunk that feeds the air handler.