About the CFM Calculator
This CFM calculator works out the airflow needed to ventilate a room. Enter the room’s length, width and ceiling height and the number of air changes per hour (ACH) you want, and it returns the required airflow in cubic feet per minute (CFM), plus cubic metres per hour and litres per second for metric fan ratings, and how often the air is fully replaced.
Use it to size a bathroom or kitchen exhaust fan, a garage or workshop extractor, a grow-room or server-closet fan, or an air purifier (whose CADR is also measured in CFM). A chart shows the airflow needed at different ventilation rates so you can compare a basic and a more aggressive setup.
The Home Ventilating Institute (HVI) suggests about 8 air changes per hour for bathrooms and 15 for kitchens; for a standard bathroom with an 8 ft ceiling that works out close to its simpler rule of 1 CFM per square foot. Duct length, bends and grilles reduce a fan’s real output, so choose a fan rated above the calculated figure at the static pressure of your installation.
With the default inputs, the required airflow is 85.3 CFM. Change any value above to recalculate instantly.
How to use the cfm calculator
- 1Measure the room length, width and ceiling height and choose feet or meters.
- 2Enter the air changes per hour you want (about 8 for a bathroom, 15 for a kitchen).
- 3Read the required CFM (and m³/h or L/s for metric fans).
- 4Buy a fan rated above that figure at your installation’s static pressure.
Formula and method
Air changes per hour is how many times the entire volume of air in a room is replaced each hour. Multiplying the room volume (length × width × height) by ACH gives cubic feet per hour; dividing by 60 converts to cubic feet per minute, the unit fans are rated in.
Metric dimensions are converted at 1 m³ = 35.3147 ft³. The airflow is also shown as m³/h (1 CFM = 1.699 m³/h) and litres per second (1 CFM = 0.472 L/s). The time for one complete air change is 60 ÷ ACH minutes. The result assumes good mixing; real fans deliver less than their rating once duct resistance is added.
- CFM
- Airflow in cubic feet per minute
- Volume
- Length × width × height of the room
- ACH
- Air changes per hour
Worked examples
10 × 8 ft bathroom, 8 ft ceiling, 8 ACH
Volume = 10 × 8 × 8 = 640 ft³. 640 × 8 ÷ 60 ≈ 85.3 CFM, so a 90 CFM bathroom fan is a good fit; the air is replaced every 7.5 minutes.
12 × 15 ft kitchen with 9 ft ceiling, 15 ACH
12 × 15 × 9 = 1,620 ft³. At 15 air changes per hour: 1,620 × 15 ÷ 60 = 405 CFM of exhaust.
Two-car garage workshop, 24 × 24 × 10 ft, 6 ACH
5,760 ft³ × 6 ÷ 60 = 576 CFM, which equals 1 CFM per square foot of floor.
Metric room 4 × 5 × 2.5 m, 6 ACH
50 m³ × 6 = 300 m³/h, which is 83.3 L/s or about 176.6 CFM (50 m³ = 1,765.7 ft³).
Frequently asked questions
How many CFM do I need for a bathroom fan?+
HVI recommends about 1 CFM per square foot of floor for bathrooms up to 100 sq ft, with a minimum of 50 CFM. A 5 × 8 ft bathroom needs 50 CFM; a 10 × 10 ft bathroom about 100 CFM.
How do I convert air changes per hour to CFM?+
Multiply the room volume in cubic feet by the ACH and divide by 60. A 1,000 ft³ room at 6 ACH needs 1,000 × 6 ÷ 60 = 100 CFM.
What is a good ACH for a room?+
Ordinary living spaces are often ventilated at 0.35–1 ACH for fresh air, bathrooms about 8 and kitchens about 15 during use. Workshops, grow rooms and smoking areas often need much higher rates.
Why does my fan not move its rated CFM?+
Fan ratings are measured at a specified static pressure. Long ducts, bends, small duct diameters and grilles add resistance, so the real airflow is lower. Check the fan’s performance at 0.25 in. w.g. or higher.
How many CFM is 1 m³/h?+
One cubic metre per hour equals about 0.589 CFM, and 1 CFM equals about 1.699 m³/h or 0.472 litres per second.