CFM Calculator
Size a fan or ventilation system in cubic feet per minute. Enter the room length, width, and ceiling height in feet, choose a target air changes per hour (ACH), and get the required CFM plus the metric m³/h equivalent.
Example: with Room length (ft) 20 · Room width (ft) 15 · Ceiling height (ft) 8 · Target air changes per hour 6 ACH — living areas → Required airflow: 240 CFM.
- Room volume2,400 cu ft
- Metric equivalent408 m³/h
- Air change intervalEvery 10 minutes
Computed by the calculator below using its default values. Change any input to see your own numbers.
CFM = room volume × air changes per hour ÷ 60. A 20 × 15 room with 8 ft ceilings at 6 ACH needs 240 CFM.
What CFM actually measures
CFM is cubic feet of air moved per minute. To ventilate a space you need enough CFM to replace the room's whole air volume several times an hour — that rate is the air changes per hour, or ACH. The math is direct: room volume × ACH gives cubic feet per hour, and dividing by 60 gives CFM. A 20 × 15 room with an 8-foot ceiling holds 2,400 cubic feet; at 6 ACH it needs 2,400 × 6 ÷ 60 = 240 CFM.
Recommended ACH depends on how much moisture, heat, and odor a room generates. Bedrooms do fine at 5–6; kitchens and bathrooms want 8 because steam and cooking byproducts need to leave quickly.
Rated CFM vs delivered CFM
A fan's box rating is measured at zero static pressure — free air, no ducts. Real installs push air through ducts, elbows, and louvers, which can knock 20–40% off delivered flow. If a bathroom needs 80 CFM through a long duct run, installing a 110 CFM fan is the practical choice. For whole-house HVAC sizing, this room-by-room ACH method is a screening estimate; a proper Manual J/D design accounts for loads and duct losses.
How it’s calculated
CFM = (length × width × height) × ACH ÷ 60, all dimensions in feet. Metric conversion: 1 CFM = 1.699 m³/h. Air change interval = 60 ÷ ACH minutes. ACH presets follow common HVAC ventilation rules of thumb by room type.
Assumes an empty rectangular room and a fan delivering its rated flow — duct static pressure typically cuts delivered CFM 20–40%, so round up when picking equipment.
Typical target air changes per hour
| Space | Recommended ACH |
|---|---|
| Bedrooms | 5–6 |
| Living areas / offices | 6–8 |
| Kitchens | 7–8 |
| Bathrooms / laundry | 8 |
| Basements / storage | 3–4 |
| Workshops (dust or fumes) | 8–10 |
Common HVAC industry ventilation rules of thumb; code minimums (ASHRAE 62.2) are set separately and are usually lower.
Common mistakes
- Using floor area instead of volume — ceiling height is a full factor in the airflow requirement.
- Sizing a bathroom or kitchen at living-room ACH, which leaves steam and odors lingering.
- Buying a fan whose rated CFM exactly matches the requirement, then losing 30% to duct resistance.
- Forgetting makeup air: a fan can only exhaust as fast as replacement air can get in.
Frequently asked questions
What is the CFM formula?
CFM = room volume × air changes per hour ÷ 60, where volume is length × width × height in feet. A 2,400 cu ft room at 6 ACH needs 240 CFM.
How many CFM do I need for a bathroom fan?
The HVI rule is 1 CFM per square foot of bathroom, with a 50 CFM minimum. An 8 × 10 bathroom needs about 80 CFM — which matches the 8 ACH figure this calculator gives for a standard 8-foot ceiling.
Is more CFM always better?
No. Oversized fans cost more, make more noise, and can depressurize a tight house enough to backdraft combustion appliances. Pick the target ACH for the room type and add a modest margin for duct losses.
What is the difference between CFM and ACH?
CFM is an airflow rate; ACH is how many times per hour that flow replaces the room's air. They are linked by room volume: ACH = CFM × 60 ÷ volume. The same 240 CFM fan gives 6 ACH in a 2,400 cu ft room but only 3 ACH in a 4,800 cu ft room.