How Many CFM Do I Need for a Room? Air Changes and Duct Size
Part of the project planner: Planning heating, cooling and ventilation.
CFM is cubic feet per minute. For a room, CFM = volume in cubic feet × air changes per hour ÷ 60. A 15 × 12 ft room with an 8 ft ceiling is 1,440 cubic feet, and changing its air 6 times an hour needs 144 CFM and a duct about 6.6 inches across. The CFM calculator does your room.
CFM by room
With a typical air-change target for each kind of room (8 ft ceilings; the duct is the round size at 600 ft/min):
| Room | Volume | Air changes per hour | CFM | Duct diameter |
|---|---|---|---|---|
| Bathroom 5 × 8 ft | 320 ft³ | 8 | 43 CFM | 3.6 in |
| Bathroom 8 × 10 ft | 640 ft³ | 8 | 85 CFM | 5.1 in |
| Bedroom 12 × 12 ft | 1,152 ft³ | 4 | 77 CFM | 4.8 in |
| Living room 15 × 20 ft | 2,400 ft³ | 4 | 160 CFM | 7.0 in |
| Kitchen 12 × 15 ft | 1,440 ft³ | 15 | 360 CFM | 10.5 in |
| Workshop 20 × 20 ft | 3,200 ft³ | 6 | 320 CFM | 9.9 in |
For particular rooms there are special rules: the bathroom fan calculator, the range hood calculator and the air purifier calculator each apply them.
Round duct size for each airflow
The diameter needed to carry the air at three speeds. Quieter systems use lower speeds and larger ducts; round up to the next standard size:
| Airflow | At 400 ft/min | At 600 ft/min | At 800 ft/min |
|---|---|---|---|
| 50 CFM | 4.8 in | 3.9 in | 3.4 in |
| 80 CFM | 6.1 in | 4.9 in | 4.3 in |
| 110 CFM | 7.1 in | 5.8 in | 5.0 in |
| 150 CFM | 8.3 in | 6.8 in | 5.9 in |
| 200 CFM | 9.6 in | 7.8 in | 6.8 in |
| 300 CFM | 11.7 in | 9.6 in | 8.3 in |
| 400 CFM | 13.5 in | 11.1 in | 9.6 in |
| 600 CFM | 16.6 in | 13.5 in | 11.7 in |
Why the real airflow is lower
- A fan's CFM is measured with no duct. Long runs, elbows, grilles and flexible duct add resistance.
- A fan rated at 110 CFM may deliver 70 or 80 CFM through a long, bent duct, so choose one rated for more, or keep the run short.
- Pick a quiet fan (a low sone rating), or you will not use it.
- Make up air has to come from somewhere: a door gap or vent, or the fan will starve.
A 110 CFM fan in the example room gives 4.6 air changes an hour. Estimates only: check your local code for required ventilation rates.
Frequently asked questions
How do you calculate CFM for a room?
Multiply the room volume in cubic feet by the air changes per hour and divide by 60. A 15 × 12 × 8 ft room (1,440 ft³) at 6 air changes an hour needs 144 CFM.
How many air changes per hour do I need?
Commonly 6 to 8 for a bathroom, 15 or more for a kitchen with a hood, and 2 to 4 for living areas. Guidance varies, so check your code.
What size duct for a given CFM?
Divide the CFM by the air speed to get the area, then the diameter. 144 CFM at 600 ft/min needs a duct 6.6 inches across, so use the next size up.
How many air changes does a 110 CFM fan give?
4.6 air changes an hour in a 1,440 ft³ room.
Does the duct run change the CFM?
Yes. A fan's CFM rating is at no resistance, and every elbow, long run and flexible duct cuts the flow, so allow a margin and keep the duct short and straight.
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Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and follow local building codes for anything structural.