TradesHow this works
The system moves a total airflow, roughly 400 CFM per ton. Each room should get a slice of that in proportion to its share of the whole-house load. This tool takes the room load and the total load, works out the room CFM, and divides it into supply registers. Balance by load and the house stays even, room to room.
This is a planning rule of thumb. A full Manual D duct design accounts for duct runs, fittings, and the blower curve. Use this to set targets and to catch rooms that are starved for air.
Common questions
Why balance by load, not by square footage?
A room with lots of west-facing glass carries far more cooling load than a shaded room of the same size. Splitting air by floor area starves the hot rooms and overcools the easy ones. Splitting it by each room's share of the load is what actually keeps the house even.
How much air does one register move?
It depends on the register size, the throw, and the static pressure, but a common residential supply register moves on the order of 100 CFM. Bigger rooms usually want two or more smaller registers rather than one large one, for better mixing and less noise.
Do I size the return too?
Yes. Air you push into a room has to get back to the system. Undersized or missing returns pressurize rooms, push conditioned air out, and starve the blower. Plan return paths along with the supplies.
Sources: Airflow rules of thumb: ~400 CFM/ton, room airflow proportional to load share; confirm with Manual D. Engine version 1.0.0. Risk tier 2.