CFM Calculator

CFM stands for Cubic Feet per Minute - a measure of airflow volume. It tells you how many cubic feet of air move past a point each minute. The basic formula is: CFM = Room Volume (cubic feet) × Air Changes per Hour (ACH) / 60. Room volume = Length × Width × Height. ACH (air changes per hour) is the number of times the total air volume in a room should be replaced per hour, which varies by room type: bathrooms need 8 ACH, kitchens need 15 ACH, living areas need 4-6 ACH.

Bathroom fan sizing: For bathrooms up to 100 sq ft, use 1 CFM per square foot of floor area. For example, a 50 sq ft bathroom needs at least 50 CFM. For bathrooms over 100 sq ft, add CFM for fixtures: Each toilet adds 50 CFM, each shower adds 50 CFM, each bathtub adds 50 CFM. Minimum: 50 CFM regardless of size. HVI (Home Ventilating Institute) recommends sizing for 8 air changes per hour. A 10×8×8 bath = 640 cu ft × 8 ACH / 60 = 85.3 CFM minimum.

HVAC systems are typically sized at 400 CFM per ton of cooling. So a 3-ton AC system needs 1,200 CFM total airflow. Room-by-room: CFM = (Room Sq Ft × Ceiling Height × ACH) / 60. For comfort cooling: 4-6 ACH is typical. For the whole house: Total CFM = (House Volume × Desired ACH) / 60. Then divide by 400 to get required tons: Tons = Total CFM / 400. Ductwork must be sized to deliver the required CFM at acceptable static pressure (typically 0.1-0.2 inches per 100 ft).

CFM (Cubic Feet per Minute) measures total volumetric airflow rate. FPM (Feet per Minute) measures air velocity - how fast the air is moving. They are related: CFM = FPM × Duct Cross-Sectional Area (sq ft). For a 12×12 inch duct (1 sq ft), air moving at 500 FPM delivers 500 CFM. CFM is used for sizing fans and equipment. FPM is used for duct design (optimal range: 300-900 FPM for main ducts, 600-900 FPM for branch ducts). Too low FPM means ducts are oversized, too high means noisy inefficient operation.