CFM per Square Foot: Estimate Airflow
What does 1 CFM per square foot mean?
CFM per square foot is a ratio of airflow to floor area. It does not define a required airflow for every room. The familiar 1 CFM/ft² shortcut can be derived from a specific example: at 8 ft high, a room volume is 8 ft³ per ft² of floor; applying 7.5 air changes per hour gives 8 × 7.5 ÷ 60 = 1 CFM per ft².
The result changes if the chosen air-change rate or ceiling height changes. Use it only as a first-pass estimate when those assumptions fit your task. For a real heating or cooling design, room airflow should follow the calculated load and the equipment and system design.
Calculate CFM from area, ceiling height, and ACH
Use consistent units: floor area in square feet, ceiling height in feet, and airflow in cubic feet per minute.
Q (CFM) = area (ft²) × ceiling height (ft) × target ACH ÷ 60
| Floor area | Ceiling height | Illustrative target | Estimated total airflow |
|---|---|---|---|
| 150 ft² | 8 ft | 7.5 ACH | 150 CFM |
| 300 ft² | 10 ft | 7.5 ACH | 375 CFM |
| 28 m² | 3 m | 7.5 ACH | about 371 CFM (about 630 m³/h) |
For a quick adjustment from the 1 CFM/ft² example, multiply by actual ceiling height ÷ 8 ft, provided you keep the same 7.5 ACH assumption. For other targets or room conditions, use the full formula instead of the shortcut.
Supply airflow is different from outdoor-air ventilation
This estimate describes total airflow through a room. A central system may recirculate much of that air. It does not tell you how much outdoor air a building needs, nor does a return-air path count as outdoor air.
Outdoor-air ventilation depends on space use, area, occupancy, the system, and locally adopted requirements. ASHRAE Standard 62.1-2025 applies to occupied spaces except nontransient residential dwelling units; Standard 62.2-2025 covers those dwelling units. Check local codes and any other applicable standards instead of substituting one CFM/ft² value. Kitchen and bathroom exhaust are separate design needs.
Continue from airflow to duct sizing
Once you have a preliminary airflow value, the duct sizing calculator can estimate an initial duct size from airflow and selected velocity. Its result is a sizing starting point; it does not model the full duct layout, fitting losses, available static pressure, or room-by-room load design.
400 ÷ 900 = 0.444 ft². An equivalent round diameter is √(4 × 0.444 ÷ π) × 12 ≈ 9.0 in, so start by checking a 9-inch round option. Then verify the run against friction rate, fittings, flex compression, noise, available static pressure, and the actual equipment airflow. Use the duct sizing calculator to repeat the calculation with your project inputs and the 0.10 in. w.c. friction-rate chart for a separate screening check.Frequently asked questions
How many CFM per square foot do I need?
There is no single value for every room. As an illustration, 7.5 ACH at an 8 ft ceiling works out to 1 CFM/ft². Choose airflow from the room's design conditions and applicable requirements.
Is 1 CFM per square foot an ASHRAE requirement?
No. It is an arithmetic shortcut for the stated 7.5 ACH and 8 ft assumptions. It does not replace an applicable ventilation standard or local code.
How do I convert CFM per square foot to CFM per square meter?
Multiply by 10.764: 1 CFM/ft² equals about 10.764 CFM/m², or 18.29 m³/h per m².
How does ceiling height change the estimate?
For the same target ACH and floor area, airflow scales with ceiling height. Use Q = area × height × ACH ÷ 60; the quick factor is actual height divided by 8 ft when starting from the 1 CFM/ft² example.
Does this estimate calculate outdoor-air ventilation?
No. Total supply airflow and required outdoor air are different quantities. Determine ventilation and local exhaust separately under the applicable standard and local code.