CFM Calculator: Cubic Feet per Minute Airflow
Enter your room dimensions and air-change rate to find out how many cubic feet per minute (CFM) of airflow you need. Switch to velocity mode to calculate CFM from duct area and air speed, or use reverse mode to see what ACH your current equipment delivers. Results update instantly in imperial or metric units.
What is CFM and why does it matter?
CFM, or cubic feet per minute, is the standard unit used in North American HVAC engineering to describe how much air moves through a space, duct, or piece of equipment in one minute. Sizing a fan, air handler, or ventilation system to the right CFM is essential: too low and the space is stuffy, humid, or accumulates pollutants; too high wastes energy, creates noise, and can cause pressure imbalances that make doors hard to close. Residential forced-air systems typically move 400-450 CFM per ton of cooling capacity, while exhaust fans in bathrooms are sized by CFM to meet local building codes.
The three CFM formulas explained
The most common formula is the ACH (air changes per hour) method: CFM = (room volume in ft³ x ACH) / 60. This tells you how many cubic feet per minute the fan must move to cycle the entire room air a given number of times each hour. The velocity method - CFM = duct area (ft²) x air velocity (ft/min) - is used by duct designers and HVAC technicians to verify that existing ductwork can carry the required flow at an acceptable speed. A third approach, the reverse calculation, works backwards from equipment rated CFM to find out what ACH that unit actually delivers in a specific room: ACH = (CFM x 60) / room volume. All three modes are available in this calculator.
Duct sizing and velocity guidelines
Once you know the required CFM, you can size a round duct by dividing CFM by the target face velocity (typically 600-900 ft/min for supply and 500-700 ft/min for return) and solving for the cross-sectional area: Area = CFM / Velocity. A 6-inch round duct carries about 80 CFM at 800 ft/min; an 8-inch duct handles around 170 CFM; a 10-inch handles about 300 CFM. Excessive velocity (above 1,200 ft/min in supply runs) causes noise and increases static pressure losses, reducing system efficiency.
Metric conversion and international use
Outside North America, airflow is usually stated in cubic metres per hour (m³/h) or litres per second (L/s). The conversions are straightforward: 1 CFM = 1.699 m³/h and 1 CFM = 28.317 L/min. This calculator automatically shows m³/h and L/min alongside every CFM result so you can cross-check equipment rated in either system. Switching to metric mode lets you enter room dimensions in metres and velocity in m/s, and the tool converts internally before displaying CFM.
Recommended ACH by room type
| Room / Application | Recommended ACH | Notes |
|---|---|---|
| Living room | 3-4 | Basic comfort ventilation |
| Bedroom | 5-6 | Sleep quality; CO2 dilution |
| Bathroom | 7-8 | Moisture and odor removal |
| Kitchen | 7-8 | Cooking fumes and steam |
| Laundry room | 8-9 | Humidity control |
| Basement | 3-4 | Radon dilution; moisture |
| Attic (cooling) | 12-15 | Summer heat extraction |
| Garage (work use) | 20-30 | Exhaust fumes removal |
| Office / open plan | 6-8 | CO2 dilution for occupants |
| Classroom | 6-20 | Depends on occupancy density |
| Restaurant dining | 8-60 | Higher end for open kitchens |
| Copy/print room | 10-12 | VOC and ozone removal |
| Hallway / corridor | 3-5 | Low occupancy |
| Swimming pool area | 10-15 | Chloramine control |
| Theatre / auditorium | 8-15 | High occupancy |
| Pharmaceutical cleanroom | 60-600 | ISO class dependent |
Typical air-change-per-hour targets used by ASHRAE and HVAC engineers. Use the lower end for energy efficiency, the upper end for higher occupancy or moisture control.
Frequently asked questions
What CFM do I need for a bedroom?
A typical bedroom targets 5-6 air changes per hour. For a 12 ft x 10 ft room with a 9 ft ceiling (1,080 ft³), that works out to (1,080 x 6) / 60 = 108 CFM. A standard 110 CFM bathroom-style exhaust fan comfortably covers most bedrooms at that ACH target, though the actual unit serving a bedroom is usually the central HVAC return, not a standalone fan.
How much CFM per square foot do I need?
At a standard 8 ft ceiling height: 1 ACH needs 0.13 CFM/sq ft, 2 ACH needs 0.27, 3 ACH needs 0.40, 4 ACH needs 0.53, 6 ACH needs 0.80, and 8 ACH needs 1.07 CFM/sq ft. These figures scale linearly with ceiling height, so a 10 ft ceiling needs 25% more CFM per square foot than an 8 ft ceiling at the same ACH target.
What is ACH and how do I choose the right value?
ACH (air changes per hour) is the number of times per hour the full volume of a room is replaced with fresh or conditioned air. Residential living areas typically use 3-6 ACH, kitchens and bathrooms 7-8 ACH, and garages or industrial spaces 20-60 ACH. Your local building code and ASHRAE Standard 62.2 (residential) or 62.1 (commercial) publish minimum requirements. When in doubt, use the higher end of the recommended range for occupant health and comfort.
What is the CFM formula?
CFM = (Volume in ft³ x ACH) / 60. Alternatively, CFM = Duct area (ft²) x Air velocity (ft/min) for velocity-based sizing. To reverse-solve: ACH = (CFM x 60) / Volume.
How do I convert CFM to m³/h?
Multiply CFM by 1.699 to get cubic metres per hour. For example, 200 CFM x 1.699 = 339.8 m³/h. To go the other direction, divide m³/h by 1.699.
What CFM fan do I need for a bathroom?
The HVI (Home Ventilating Institute) recommends a minimum of 1 CFM per square foot of bathroom floor area for bathrooms up to 100 sq ft, so a 60 sq ft bathroom needs at least a 60 CFM fan. For bathrooms larger than 100 sq ft, add 50 CFM per fixture (toilet, shower, bath). Many building codes require a minimum 50 CFM regardless of room size, and 110 CFM fans are common for comfort.