~80 CFM · 5" duct
Required Airflow
80
CFM
5" round duct @ 700 fpm
Room Volume
960
ft³
Fan Capacity
80–96
CFM
How we calculated this
= 960 ft³
= 80 CFM
diameter ≈ 4.58″ → 5″ standard
Estimate based on standard room ACH targets. For ducted systems, also account for static pressure and duct run length.
Reference
CFM by Room Type
Standard ACH targets and example CFM for a 100 sq ft room with 8 ft ceilings.
| Room Type | Recommended ACH | Example CFM (100 sq ft × 8 ft) |
|---|---|---|
| Bedroom | 5–6 | 67–80 |
| Living Room | 6–8 | 80–107 |
| Kitchen | 7–8 | 93–107 |
| Bathroom | 8–10 | 107–133 |
| Office | 6–8 | 80–107 |
| Garage | 6–8 | 80–107 |
| Workshop | 10–12 | 133–160 |
Learn
Understanding Airflow & CFM
What CFM is
CFM is cubic feet per minute. The volume of air a fan, vent, or duct moves in a minute. It's the unit HVAC techs use to size ductwork, exhaust fans, and air handlers. More CFM, more air movement.
The formula
Required CFM is room volume times air changes per hour, divided by 60: CFM = (L × W × H × ACH) / 60. Take a 12 × 10 bedroom with 8 ft ceilings (960 ft³) at 5 ACH, and you get (960 × 5) / 60 = 80 CFM.
Why room type changes ACH
ACH is how many times the room's full air volume gets replaced per hour. Rooms don't all need the same rate. Bedrooms turn the air over 5–6 times an hour because the people in them aren't moving much. Kitchens need 7–8 ACH and bathrooms 8–10 to clear cooking fumes, steam, and odors before they spread. Workshops and garages push to 10–12 to deal with dust, paint vapors, and combustion byproducts.
CFM vs. static pressure
CFM is how much air has to move; static pressure is how hard the duct resists it. Long runs, sharp turns, and undersized ducts all drive static pressure up and choke the flow. A 6" round duct at 700 fpm carries about 140 CFM cleanly. Push past that and noise, vibration, and fan strain climb fast.
When to size up airflow
Add 10–20% for vaulted ceilings over 9 ft, rooms with real heat sources like computers, ovens, or west-facing glass, and high-occupancy spaces such as home theaters and gyms. Rooms with poor return-air paths benefit too, since a pressure imbalance eats into effective airflow.
FAQ
Common Questions About CFM Sizing
How do I calculate CFM for a room?
Multiply length by width by height for the room's volume in cubic feet, multiply that by the room's target air changes per hour (ACH), then divide by 60. The answer is your CFM. A 12 × 10 × 8 bedroom at 5 ACH works out to (960 × 5) / 60 = 80 CFM.
What is a good CFM for a bedroom?
Most bedrooms land between 60 and 100 CFM. They sit at the low end of the ACH scale (5–6) because occupants are stationary and air-quality demands are modest. A 100 sq ft bedroom with 8 ft ceilings needs roughly 67–80 CFM; a 200 sq ft master needs around 130–160.
How many CFM per square foot?
The usual rule of thumb is 1 CFM per square foot for general residential spaces with 8 ft ceilings, which comes out to about 7–8 ACH. Bedrooms can run lower (0.7 CFM/sq ft); kitchens and bathrooms run higher (1.2–1.5). The shortcut falls apart with non-standard ceiling heights, so for exact sizing use volume and ACH instead.
What's the difference between CFM and ACH?
ACH (air changes per hour) is a ratio: how many times the room's entire air volume gets swapped out in an hour. CFM is the raw flow rate in cubic feet per minute. You design to an ACH target for the room type, then spec the equipment in CFM. CFM = Volume × ACH / 60 moves between the two. If you already know your CFM and want to check the ACH, rearrange it to ACH = CFM × 60 / Volume.
Does duct size affect CFM?
Yes. The duct has to carry the required CFM at a sane air velocity, typically 600–900 fpm in a home. Undersize it and the fan works harder: static pressure climbs, so do noise and energy use, and the air that actually reaches the room drops. A 6" round duct handles 100–150 CFM cleanly at 700 fpm. Past that, step up to 7" or 8".
Related Tools
Duct Size Calculator
Convert CFM into round or rectangular duct dimensions at your target velocity.
↳ 80 CFM @ 700 fpm → 5" round
BTU Calculator
Pair airflow sizing with cooling capacity for the same room.
↳ 12×10 room → ~5,000 BTU
ACH Calculator
Compute air changes per hour from a known CFM and room volume.
↳ 80 CFM · 960 ft³ → 5 ACH