CFM Calculator — Airflow and Air Changes
Converts between room size, air changes and airflow. Use the first mode to size ventilation for a room, and the second to find the airflow a cooling system of a given capacity needs.
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The formula
CFM = room volume (ft³) × air changes per hour ÷ 60 CFM = cooling tons × 400
CFM is cubic feet per minute, and air changes per hour is a rate per hour, so the only real step is dividing by 60. The 400 CFM per ton figure is a design convention rather than a law of physics — it is the airflow at which a standard coil removes both the right amount of heat and the right amount of moisture.
What to watch out for
The two modes answer different questions:
- Air changes per hour is a ventilation figure — how often the room's air is replaced. It sets fan and extract sizing, and it is what building regulations tend to specify.
- 400 CFM per ton is a cooling figure. It is the airflow a coil needs to do its job properly. Too little and the coil ices up; too much and the air leaves the coil warm and wet.
They are not interchangeable. A room can meet its air-change requirement and still need far more airflow to stay cool.
Frequently Asked Questions
How do I calculate CFM for a room?
Multiply the room volume in cubic feet by the air changes per hour you need, then divide by 60. A 12 × 15 ft room with an 8 ft ceiling is 1,440 ft³; at 6 air changes per hour that is 144 CFM.
How many CFM per ton of air conditioning?
400 CFM per ton is the standard design figure, so a 3-ton system moves about 1,200 CFM. Dry climates sometimes use 450 for more sensible cooling; humid climates use 350 so the coil has longer to condense moisture out of the air.
What is a good air changes per hour?
Four to six for bedrooms, living rooms and offices; 10–15 for domestic kitchens; six to eight for bathrooms and classrooms. Higher rates are about removing moisture, smells or heat from equipment rather than about comfort.