Airflow comparison
CFM per Square Foot Calculator
Calculate a known airflow-to-area ratio. This comparison metric does not set a universal airflow or ventilation target.
Calculate CFM per ft²
Use the floor area actually served by the stated, clearly identified airflow stream.
How to use this calculator
- Identify the airflow stream: supply, outdoor, exhaust, return, or recirculated air.
- Enter its CFM and the conditioned floor area it actually serves.
- Compare the ratio only with spaces and assumptions that match.
Formula
Variables and limitations
| Input | What it provides | What it omits |
|---|---|---|
| Airflow | Known volume-flow value | Whether the flow is appropriate for its purpose |
| Floor area | Area-normalized comparison basis | Ceiling height, room volume, and exposed surfaces |
| Ratio | CFM per ft² | Ventilation, cooling-load, and duct-design requirements |
Example
240 CFM ÷ 300 ft² = 0.80 CFM per ft². A room with the same area but a higher ceiling has a different volume and may produce a different ACH at the same CFM.
Result interpretation
The ratio helps describe a stated flow, but it cannot establish an appropriate design value by itself. Ceiling height, occupants, air-stream purpose, climate, loads, outdoor-air requirements, filtration, humidity, and air distribution remain relevant.
Use a companion calculation
Use the CFM Calculator to relate a documented ACH to room volume, or the HVAC Load Calculator when airflow is being considered from a preliminary load.
Frequently asked questions
Is CFM per square foot a code requirement?
Not by itself. Applicable requirements depend on the occupancy, air stream, system, and local jurisdiction.
Can this tool size a system?
No. It only divides two values. Equipment sizing requires a documented load calculation and manufacturer performance data.
Should I use gross building area?
Use the area actually served by the input airflow and record the boundary used. Mixing conditioned and unconditioned areas makes the comparison unreliable.