CFM Calculator
Calculate cubic feet per minute (CFM) airflow based on room volume and required air changes per hour.
Typical: 4-12 ACH
Results
At 240 CFM, ensure ductwork is properly sized. A 10 to 14 inch round duct is typically suitable for this flow rate.
How to Use This Calculator
Enter the room volume by providing length, width, and ceiling height in feet. For example, a 20 by 15 foot room with a 9 foot ceiling has 2,700 cubic feet of volume. Select the room type or application, such as office, classroom, residential bedroom, or commercial kitchen, as each has different ventilation requirements per ASHRAE Standard 62.1 or 62.2. Alternatively, enter the desired air changes per hour directly if you have a specific requirement. For supply duct sizing, enter the total system CFM and the number of supply outlets. The calculator divides the total CFM among outlets and recommends duct sizes. A 2,700 cubic foot classroom requiring 6 air changes per hour needs 270 CFM of supply air, which can be delivered through three 90 CFM outlets using 6 inch round ducts each.
Understanding the Concept
CFM, or cubic feet per minute, measures volumetric airflow and is the fundamental unit for sizing HVAC distribution systems. Every component in an air distribution system, including fans, ductwork, registers, grilles, and diffusers, is selected based on CFM requirements. The required CFM for a space depends on its use and occupancy. ASHRAE Standard 62.1 establishes minimum ventilation rates for commercial buildings based on occupancy and floor area. ASHRAE Standard 62.2 covers residential ventilation. Beyond ventilation minimums, the supply CFM must also satisfy the heating or cooling load. The supply CFM for cooling is calculated from the total cooling load divided by the sensible capacity per CFM of supply air. Properly balanced CFM delivery ensures each room reaches the desired temperature without excessive drafts, stagnant zones, or pressure imbalances that cause doors to slam or whistle.
The Formula Explained
For ventilation based CFM: CFM = (Room Volume in cubic feet x Air Changes per Hour) / 60. For example, a 3,000 cubic foot room requiring 4 ACH needs (3000 x 4) / 60 = 200 CFM. For cooling load based CFM: CFM = Total Sensible Cooling Load (BTU/hr) / (1.08 x Delta T), where 1.08 is a constant derived from the density and specific heat of standard air (0.075 lb/ft3 x 0.24 BTU/lb/F x 60 min/hr), and Delta T is the temperature difference between return air and supply air, typically 18 to 22 degrees F. A room with a 9,000 BTU/hr sensible cooling load and a 20 degree F supply air differential requires 9000 / (1.08 x 20) = 417 CFM. These formulas are derived from ASHRAE Handbook of Fundamentals Chapter 21.
Frequently Asked Questions
How many CFM per ton of air conditioning?
The industry standard is 400 CFM per ton of cooling capacity. A 3 ton system should deliver 1,200 CFM. This can range from 350 to 450 CFM per ton depending on equipment design and sensible heat ratio. Operating below 350 CFM per ton risks evaporator coil icing, while above 450 CFM per ton reduces dehumidification.
How do I measure CFM in existing ductwork?
Use a balancing hood (flow capture hood) at supply registers for the most accurate measurement. Alternatively, measure air velocity with an anemometer at the duct cross section and multiply by the duct area in square feet. For a 10 inch round duct, the area is 0.545 square feet. At 700 FPM velocity, the airflow is 0.545 x 700 = 382 CFM.
What is the minimum CFM for a bedroom?
ASHRAE Standard 62.2 requires a minimum of 0.35 air changes per hour or 15 CFM per person for residential spaces, whichever is greater. For a typical 150 square foot bedroom with 8 foot ceilings occupied by one person, this equates to about 15 CFM for mechanical ventilation, plus the supply CFM needed to meet the heating or cooling load.
How does altitude affect CFM calculations?
At higher altitudes, air density decreases. At 5,000 feet elevation, air is approximately 17% less dense than at sea level. HVAC systems must deliver proportionally more CFM to move the same mass of air. Multiply the calculated CFM by an altitude correction factor: 1.0 at sea level, 1.10 at 3,000 feet, 1.17 at 5,000 feet, and 1.26 at 7,000 feet.