CFM Calculator
A 12 by 15 ft room with an 8 ft ceiling at 6 ACH needs 144 CFM. This CFM calculator uses the air changes per hour method for a simple rectangular room. Enter the room length, width, ceiling height, and target ACH to estimate the airflow a fan, grille, or ventilation run should move.
Quick answer
CFM means cubic feet per minute of airflow.
What this tells you
- •CFM means cubic feet per minute of airflow.
- •ACH means how many times the full room air volume is replaced each hour.
- •The calculator finds room volume first, then divides the hourly airflow target across 60 minutes.
- •This is a room-airflow estimate, not a full duct or equipment sizing method.
How to Use
- 1Choose whether your room dimensions are in feet or meters.
- 2Enter the inside length, width, and ceiling height of the room.
- 3Enter the target air changes per hour for the space.
- 4Calculate to see the required airflow in CFM, plus room volume in cubic feet and cubic meters.
How It Works
Formula
Room Volume = Length x Width x Height
CFM = Room Volume x ACH / 60
Metric inputs are converted to feet before the airflow stepThe calculator converts your chosen room dimensions to feet when needed, multiplies length, width, and ceiling height to get room volume, then multiplies by the target air changes per hour. Dividing by 60 minutes gives the airflow needed each minute to reach that ACH in a rectangular room.
Calculation note: values are processed in the order shown above, using the current input units.
Worked Examples
12 by 15 ft bedroom at 6 ACH
Room volume = 12 x 15 x 8 = 1,440 cubic feet. Airflow = 1,440 x 6 / 60 = 144 CFM.
4 by 5 by 2.4 m studio at 6 ACH
A 4 x 5 x 2.4 meter room has a volume of 48 cubic meters. At 6 air changes per hour, that becomes 288 cubic meters per hour, which is 4.8 cubic meters per minute or about 169.51 CFM.
20 by 24 ft garage at 8 ACH
Room volume = 20 x 24 x 10 = 4,800 cubic feet. Airflow = 4,800 x 8 / 60 = 640 CFM.
Common mistakes
- Using floor area instead of room volume. CFM from ACH needs length, width, and height.
- Entering metric dimensions while the calculator is still set to feet, or the reverse.
- Using outside building dimensions instead of the inside air volume you want to condition or ventilate.
- Treating the ACH result as final equipment sizing without checking static pressure, duct losses, filters, or heat load.