HVAC
HVAC CFM Calculator
Check airflow against equipment requirement, and against what the duct system can realistically deliver.
Result
400 CFM per ton is the common design figure, but it is a starting point. Humid climates often run 350 to remove more moisture, and heat pumps in heating mode want more. Measured static pressure, not a rule of thumb, tells you what a duct system is actually delivering.
Get a Free Business AuditWhat moves the number
- Blower capability at the measured static pressure rather than at free air.
- Return path size, which is the most commonly undersized part of a system.
- Filter resistance, which changes as the filter loads.
Where this one goes wrong
Setting blower speed up to hit a CFM target on a restricted system. It raises noise and pressure without necessarily raising delivered airflow, and it hides the duct problem rather than solving it.
Questions
About this calculator
How do I know whether airflow is actually adequate?
Measure it rather than infer it. Temperature split alone can mislead, because charge and load both affect it. Static pressure and an airflow measurement together tell you what is happening.
Why is the return usually the problem?
Because returns are often added as an afterthought and sized smaller than the supply. Air that cannot get back to the air handler cannot be delivered, no matter how good the supply side is.
Does a higher MERV filter reduce airflow?
It can, particularly in a system with little spare static pressure. The answer is usually more filter area rather than a lower rating, because a larger filter at the same rating is less restrictive.
The formula
room CFM = volume x air changes / 60; system CFM = tons x CFM per ton
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How to use this calculator
- Work it out from. Enter your own figure; the page starts at System tonnage.
- System size (tons) (tons). Enter your own figure; the page starts at 3 tons.
- CFM per ton. 400 is typical. Humid climates run lower, dry climates higher.
- Room length (ft) (ft). Enter your own figure; the page starts at 16 ft.
- Room width (ft) (ft). Enter your own figure; the page starts at 14 ft.
- Ceiling height (ft) (ft). Enter your own figure; the page starts at 8 ft.
The result updates as you type, so you can try a figure, see what it does to air flow needed, 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 |
|---|---|
| Work it out from | System tonnage |
| System size (tons) | 3 tons |
| CFM per ton | 400 |
| Room length (ft) | 16 ft |
| Room width (ft) | 14 ft |
| Ceiling height (ft) | 8 ft |
| Air changes per hour | 6 |
What comes out
| Result | Value |
|---|---|
| Air flow needed main result | 1,200 |
| From room volume | 179 |
| From tonnage | 1,200 |
| Room volume | 1,792 |
A 3 ton system moves 1,200 CFM at 400 CFM per ton.
How sensitive the answer is
Holding everything else steady and changing only system size (tons), this is what happens to air flow needed. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| System size (tons) (tons) | Air flow needed |
|---|---|
| 1.5 tons | 600 |
| 2.25 tons | 900 |
| 3 tons example above | 1,200 |
| 4.5 tons | 1,800 |
| 6 tons | 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
- Work it out from
- Choose from System tonnage, Room size and air changes.
- System size (tons) (tons)
- Measured in tons, starting at 3 tons.
- CFM per ton
- 400 is typical. Humid climates run lower, dry climates higher.
- Room length (ft) (ft)
- Measured in ft, starting at 16 ft.
- Room width (ft) (ft)
- Measured in ft, starting at 14 ft.
- Ceiling height (ft) (ft)
- Measured in ft, starting at 8 ft.
- Air changes per hour
- A number, between 0 and 60, starting at 6.
What the results tell you
| Result | What it is |
|---|---|
| Air flow needed | The headline result. A number. |
| From room volume | A number. |
| From tonnage | A number. |
| Room volume | A number. |
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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Open calculatorEstimating is the easy part
The jobs you never hear about cost more than the ones you mis-measure. BDEVY builds the marketing, systems and automation that bring them in.