How to size airflow in CFM
CFM (cubic feet per minute) is how much air a fan moves. To ventilate a room: CFM = room volume × air changes per hour ÷ 60. A 20 × 15 ft room with an 8 ft ceiling is 2,400 cubic feet; at 8 air changes per hour that needs 2,400 × 8 ÷ 60 = 320 CFM. Room volume starts from the floor area given by the square footage calculator.
How to use this calculator
Enter the room length, width and ceiling height in feet, plus the target air changes per hour (ACH) for the space. The result is the airflow your fan or vent needs to deliver. Pick a fan rated at or above this CFM. If you prefer to work in cubic feet directly, use the cubic feet calculator.
Recommended air changes per hour
| Room | ACH |
|---|---|
| Bedroom / living | 4–6 |
| Kitchen | 7–8 |
| Bathroom | 8–10 |
| Workshop / garage | 6–10 |
A quick rule for bathrooms
For small bathrooms a simpler rule is 1 CFM per square foot of floor (minimum 50 CFM). For rooms over 100 square feet, the ACH method above is more accurate. Add capacity for long duct runs, since bends and length reduce real airflow.
Continuous low-speed ventilation often beats a big intermittent fan: a 50 CFM unit running for an hour moves the same air as a 300 CFM fan for ten minutes, with far less noise and draught. The liquid equivalent of this flow measure is handled by the GPM calculator.
Frequently asked questions
- How do I calculate CFM for a room?
- Multiply room volume (L × W × H) by air changes per hour, then divide by 60. A 2,400 ft³ room at 8 ACH needs 320 CFM.
- What is a good CFM for a bathroom?
- Use about 8–10 air changes per hour, or the simple rule of 1 CFM per square foot with a 50 CFM minimum.
- What does ACH mean?
- Air changes per hour — how many times the room's full air volume is replaced each hour. Higher ACH means more ventilation.
- Should I oversize the fan?
- Slightly. Duct length and bends cut real airflow, so choosing a fan a bit above the calculated CFM helps it hit the target.
- How do I convert CFM to cubic metres per hour?
- Multiply CFM by 1.699. A 320 CFM fan moves about 544 cubic metres per hour, which is how European and metric-spec fans are usually rated.