System & Room Details
Airflow Results
There are two standard ways to calculate the airflow a system needs, and they approach the problem from different angles. The tonnage method ties airflow to the cooling capacity: every ton of AC cooling requires a certain volume of air per minute to exchange heat effectively at the coil. The air-changes-per-hour method ties airflow to the room volume: it ensures the air is refreshed often enough to remove heat, moisture and pollutants evenly. This calculator runs both and uses the higher figure, which is always the binding constraint.
| Space type | Recommended ACH | Notes |
|---|---|---|
| Bedrooms | 4 to 6 | Quiet operation preferred, lower velocity |
| Living rooms, dining | 6 to 8 | Standard residential |
| Kitchens | 8 to 12 | High heat and humidity load |
| Home offices | 6 to 8 | Equipment heat offsets lower occupancy |
| Commercial spaces | 8 to 15 | Higher occupancy and activity |
| Server rooms | 12 to 30 | Very high heat density requires rapid exchange |
| Design rule | Best for | Effect |
|---|---|---|
| 350 CFM per ton | Hot humid climates | Slower air over coil, better moisture removal |
| 400 CFM per ton | Mixed / standard | Balanced cooling and dehumidification |
| 450 CFM per ton | Hot dry climates | More airflow, faster temperature drop |
A 20 by 16 ft open-plan space with a 9 ft ceiling, served by a 1.5 ton air handler, in a standard mixed climate. Target 8 ACH for the kitchen activity.
What is CFM in HVAC?
Cubic feet per minute: the volume of air the system moves each minute. Too little starves the coil and causes icing; too much reduces efficiency and increases noise. Getting it right is as important as choosing the correct tonnage.
How many CFM per ton?
400 CFM per ton is the standard residential rule. High-humidity climates use 350; dry climates can use 450. A 1.5 ton system needs 600 CFM at 400 CFM/ton.
How do I calculate CFM from room volume?
Volume in cu ft times target ACH, divided by 60. A 2,880 cu ft room at 6 ACH needs 2,880 times 6 divided by 60 equals 288 CFM by the ACH method.
What is a good CFM per square foot?
About 1 CFM per square foot for a standard 8 to 10 ft ceiling. Taller rooms and higher-heat spaces need more per square foot.
What happens if CFM is too low?
The coil freezes over, cooling stops and the system may be damaged. Low CFM also causes uneven cooling with warm far corners and can lead to humidity problems in humid climates.
Do supply and return CFM need to match?
Yes. Unbalanced supply and return pressurises rooms, reducing overall system airflow and efficiency. Every supply register needs a return air path.
CFM estimates are based on standard HVAC design rules. Actual airflow delivered depends on duct design, equipment performance and system static pressure.