Inputs
~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 is CFM?
CFM stands for cubic feet per minute — the volume of air a fan, vent, or duct moves in one minute. It's the standard unit HVAC pros use to size ductwork, exhaust fans, and air handlers. A higher CFM means more air movement, which translates to better ventilation, faster temperature changes, and improved indoor air quality.
The CFM formula
Required CFM is room volume multiplied by air changes per hour, divided by 60: CFM = (L × W × H × ACH) / 60. A 12 × 10 bedroom with 8 ft ceilings (960 ft³) at 5 ACH needs (960 × 5) / 60 = 80 CFM.
Why room type changes ACH
Different rooms need different air-exchange rates. Bedrooms turn over the air 5–6 times per hour because occupants are stationary. Kitchens and bathrooms need 8–10 ACH to clear cooking fumes, steam, and odors fast. Workshops and garages run 10–12 ACH to handle dust, paint vapors, and combustion byproducts. Match the ACH to what the room actually does.
CFM vs. static pressure
CFM tells you how much air needs to move; static pressure tells you how hard the duct fights against that movement. Long runs, sharp turns, and undersized ducts all raise static pressure and choke airflow. A 6" round duct at 700 fpm carries roughly 140 CFM cleanly — push it harder and noise, vibration, and fan strain climb fast. Use the Duct Size Calculator to convert CFM into a properly sized duct.
When to size up airflow
Bump CFM by 10–20% for vaulted ceilings (over 9 ft), rooms with major heat sources (computers, ovens, west-facing glass), and high-occupancy spaces like home theaters or gyms. Spaces with poor return-air paths also benefit from extra capacity since pressure imbalance reduces effective airflow.
FAQ
Common Questions About CFM Sizing
How do I calculate CFM for a room?
Multiply the room's length, width, and height to get its volume in cubic feet. Multiply that by the room's target air changes per hour (ACH), then divide by 60. The result is the CFM needed. For a typical 12 × 10 × 8 bedroom at 5 ACH, that's (960 × 5) / 60 = 80 CFM.
What is a good CFM for a bedroom?
Most bedrooms need 60–100 CFM depending on size. Bedrooms run on the low end (5–6 ACH) 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 bedroom needs around 130–160 CFM.
How many CFM per square foot?
A common rule of thumb is 1 CFM per square foot for general residential spaces with 8 ft ceilings, which works out to about 7–8 ACH. Bedrooms can run lower (0.7 CFM/sq ft); kitchens and bathrooms run higher (1.2–1.5 CFM/sq ft). For exact sizing, use volume and ACH instead — the per-square-foot shortcut breaks down with non-standard ceiling heights.
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 replaced in one hour. CFM is the absolute flow rate in cubic feet per minute. ACH is the design target for a room type; CFM is what you spec the equipment to. The formula CFM = Volume × ACH / 60 converts between them. Use the ACH Calculator if you already know your CFM and want to verify ACH.
Does duct size affect CFM?
Yes — the duct must be large enough to carry the required CFM at a reasonable air velocity (typically 600–900 fpm for residential). Undersized ducts force the fan to work harder, raising static pressure, noise, and energy use, while reducing actual delivered CFM. A 6" round duct handles roughly 100–150 CFM cleanly at 700 fpm; beyond 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 → 6" 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