How this calculator works
CFM is cubic feet per minute. A room of a given volume that is to change its air a number of times an hour needs the volume times the air changes, divided by 60 minutes. Going the other way, a fan's CFM times 60, divided by the volume, gives the air changes per hour. The duct area is the CFM divided by the air speed.
Worked example: a 15 × 12 ft room with an 8 ft ceiling is 1,440 ft³. At 6 air changes an hour it needs 144 CFM, which at 600 ft/min needs a duct 6.6 inches across. A 110 CFM fan gives 4.6 air changes an hour in that room.
CFM by room size and air changes (8 ft ceiling)
Rooms of common sizes.
| Room | 4 ACH | 6 ACH | 8 ACH | 15 ACH |
|---|---|---|---|---|
| 8 × 8 ft | 34 | 51 | 68 | 128 |
| 10 × 12 ft | 64 | 96 | 128 | 240 |
| 12 × 12 ft | 77 | 115 | 154 | 288 |
| 15 × 12 ft | 96 | 144 | 192 | 360 |
| 20 × 15 ft | 160 | 240 | 320 | 600 |
Tips before you buy
- A CFM rating is measured with no duct. Every elbow and extra length reduces the real flow, so round up.
- A fan that moves enough air but is too loud will not get used. Check its sone rating.
- For a bathroom, use the rule of 1 CFM per square foot (minimum 50 CFM), or 8 air changes per hour for the room.
- For a kitchen, the hood CFM depends on the cooktop as well as the room.
- A smaller duct raises the speed and the noise. Keep the air speed in a branch duct to about 600 ft/min.
Tools & materials for this project
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Frequently asked questions
How do you calculate CFM for a room?
Room volume in cubic feet times air changes per hour, divided by 60. A 15 × 12 × 8 ft room at 6 ACH needs 144 CFM.
How many air changes per hour do I need?
About 6 to 8 for a bathroom, 15 or more for a kitchen, and 2 to 4 for living areas, depending on the guidance you follow. Occupants and use change it.
What size duct for a given CFM?
Divide the CFM by the air speed to get the area, then find the diameter. 144 CFM at 600 ft/min is a duct 6.6 inches across, so use the next size up.
How many air changes does a fan give?
CFM × 60 ÷ room volume. A 110 CFM fan gives 4.6 changes an hour in a 1,440 ft³ room.
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Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.