Room Ductwork CFM Calculator
Work out how much airflow (CFM) a room’s ducts should deliver from the system tonnage and the room’s share of the conditioned area.
How it works
Total CFM = tons × CFM per ton · Room CFM = total CFM × (room area ÷ home area) · Per register = room CFM ÷ registers
A blower moves a roughly fixed amount of air — about 400 CFM per ton of cooling — and that air is divided among the rooms in proportion to their load. Using floor area as a proxy for load, each room’s duct should carry its share of the total airflow. Splitting that across the room’s registers gives a target for balancing dampers so no room is starved or over-blown.
Worked example
A 3-ton system at 400 CFM/ton moves 1,200 CFM. A 240 ft² bedroom in a 1,600 ft² home is 15% of the area, so it should get 1,200 × 0.15 = 180 CFM — 90 CFM through each of its two registers.
Frequently asked questions
How many CFM per ton should I use?
About 400 CFM per ton is the standard for cooling; 350 CFM/ton is common in humid climates to pull out more moisture, and heat-pump heating sometimes uses more. Match the value to your equipment’s specification.
Why balance airflow room by room?
Because a far room on a long duct run naturally receives less air than one next to the air handler. Sizing each duct to the room’s share and trimming with balancing dampers evens out temperatures across the house.
Is floor area a good stand-in for load?
It is a reasonable first pass when rooms have similar ceilings, insulation and sun. A room with lots of west-facing glass or an exposed roof will need more than its area share, so adjust upward for those.
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This calculator is for educational purposes. Double-check important results before acting on them.