How to Calculate CFM for a Room (HVAC Airflow, Explained)
The simple formula: room volume × air changes ÷ 60 — with real examples
CFM — cubic feet per minute — is the number every HVAC job starts with. Get it wrong and nothing else matters: undersized ducts starve the system, oversized ones make it noisy and short-cycle. The good news: calculating it takes one formula and a tape measure.
The formula
CFM = (Room sq ft × Ceiling height × Air changes per hour) ÷ 60
That's it. Room volume (length × width × height), multiplied by how many times per hour you want that air replaced, divided by 60 to get per-minute.
What ACH should you use?
Air changes per hour (ACH) is the target that makes the formula work:
| Space | ACH |
|-------|-----|
| Living rooms, bedrooms | 3–4 |
| Kitchens | 6–8 |
| Bathrooms (with exhaust) | 6–8 |
| Basements | 2–3 |
Worked examples
Example 1: 400 sq ft living room, 9-ft ceilings, 4 ACH
(400 × 9 × 4) ÷ 60 = 240 CFM
Example 2: 150 sq ft kitchen, 9-ft ceilings, 8 ACH
(150 × 9 × 8) ÷ 60 = 180 CFM
Example 3: 300 sq ft bedroom, 8-ft ceilings, 3 ACH
(300 × 8 × 3) ÷ 60 = 120 CFM
The rule of thumb
For standard 8–9 ft ceilings, roughly 1 CFM per square foot lands you in the right neighborhood — but always run the real formula for a room you're sizing ductwork for.
Common mistakes
Do the math in seconds
Use the free [HVAC CFM calculator](https://prybar.ai/tools/hvac-cfm-calculator) — enter room size, ceiling height, and air changes, and it gives you the required CFM instantly. Pair it with the [duct sizing calculator](https://prybar.ai/tools/duct-sizing-calculator) to turn that CFM into the right round duct diameter, or the [ACH calculator](https://prybar.ai/tools/ach-calculator) to check an existing system.
Beyond the math
CFM is the starting point, not the whole load calculation — BTUs, duct runs, and static pressure all matter for a real design. But this formula is what separates "feels okay" from properly sized, and it's the number customers expect you to know cold.
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