HVAC
CFM Calculator
Work out the airflow a space or a system needs, and check it against what the equipment and ducts can actually deliver.
Result
Four hundred CFM per ton is the standard cooling design figure. Duct area assumes 700 feet per minute, typical for branch ducts. Trunk lines run faster and returns slower.
Get a Free Business AuditWhat moves the number
- Required airflow per ton of capacity, which varies by equipment and by climate.
- External static pressure, which is what actually limits delivered airflow.
- Filter and coil resistance, which rise as they load up.
Where this one goes wrong
Calculating required CFM without measuring static pressure. The requirement is a target; the system delivers whatever the pressure allows, and most underperforming systems are restricted rather than undersized.
Before you quote
- Measured total external static pressure against the equipment's rating.
- Whether the return is as large as the supply, which it often is not.
- Filter type, since a restrictive filter can consume much of the available pressure.
Questions
About this calculator
Is 400 CFM per ton a reliable figure?
It is a common starting point, not a rule. Humid climates often run lower to improve moisture removal, and the manufacturer's data for the specific equipment is what should be used.
Why measure static pressure?
Because it is the difference between the airflow you specified and the airflow the system delivers. A system that cannot move its rated air will underperform regardless of how well it was sized.
What if the ducts cannot carry the required airflow?
Then the duct system is the job, not the equipment. Fitting larger equipment to restricted ducts makes the problem worse, because the airflow deficit grows with capacity.
The formula
CFM = tons × 400, or (room volume × ACH) ÷ 60
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How to use this calculator
- Method. Enter your own figure; the page starts at By system tonnage.
- System size (tons). Enter your own figure; the page starts at 3.
- CFM per ton. Enter your own figure; the page starts at 400.
- Room area (sq ft) (sq ft). Enter your own figure; the page starts at 300 sq ft.
- Ceiling height (ft) (ft). Enter your own figure; the page starts at 9 ft.
- Air changes per hour. Enter your own figure; the page starts at 6.
The result updates as you type, so you can try a figure, see what it does to airflow required, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Method | By system tonnage |
| System size (tons) | 3 |
| CFM per ton | 400 |
| Room area (sq ft) | 300 sq ft |
| Ceiling height (ft) | 9 ft |
| Air changes per hour | 6 |
What comes out
| Result | Value |
|---|---|
| Airflow required main result | 1,200 |
| Room volume | 2,700 |
| Resulting air changes per hour | 26.7 |
| Duct area needed | 246.9 |
You need 1,200 CFM, which is 26.7 air changes per hour in a 2,700 cubic foot space and needs roughly 246.9 square inches of duct at 700 fpm.
How sensitive the answer is
Holding everything else steady and changing only system size (tons), this is what happens to airflow required. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| System size (tons) | Airflow required |
|---|---|
| 1.5 | 600 |
| 2.25 | 900 |
| 3 example above | 1,200 |
| 4.5 | 1,800 |
| 6 | 2,400 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Method
- Choose from By system tonnage, By air changes per hour.
- System size (tons)
- A number, between 0 and 100, starting at 3.
- CFM per ton
- A number, between 0 and 600, starting at 400.
- Room area (sq ft) (sq ft)
- Measured in sq ft, starting at 300 sq ft.
- Ceiling height (ft) (ft)
- Measured in ft, between 0 and 30, starting at 9 ft.
- Air changes per hour
- A number, between 0 and 60, starting at 6.
What the results tell you
| Result | What it is |
|---|---|
| Airflow required | The headline result. A number. |
| Room volume | A number. |
| Resulting air changes per hour | A decimal figure. |
| Duct area needed | A decimal figure. |
Before you rely on this number
Sizing, capacity and load figures here are planning estimates. A real installation depends on the manufacturer's data for the specific equipment, the duct system it is going into, and a proper load calculation for the building. Use this to sanity-check a number or price a job, not to select equipment.
Keep going
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