CFM Calculator
Estimate cfmcalculator for your project with our free calculator. Get accurate material quantities, costs, and specifications.
Formula
CFM = (ACH x Volume) / 60 | CFM = Velocity (FPM) x Area (sq ft)
There are two common methods. From room requirements: multiply the target air changes per hour by the room volume in cubic feet, then divide by 60. From duct measurements: multiply the air velocity in feet per minute by the duct cross-sectional area in square feet.
Worked Examples
Example 1: Bedroom Ventilation CFM
Problem: Find the CFM needed for a 20 x 15 x 9 ft room with 6 ACH.
Solution: Volume = 20 x 15 x 9 = 2,700 cu ft\nCFM = (6 x 2,700) / 60\nCFM = 16,200 / 60 = 270
Result: 270 CFM required
Example 2: CFM from Duct Velocity
Problem: A 12-inch round duct has 800 FPM velocity. What is the CFM?
Solution: Radius = (12/12) / 2 = 0.5 ft\nArea = 3.14159 x 0.5^2 = 0.7854 sq ft\nCFM = 800 x 0.7854 = 628.3
Result: 628.3 CFM through the duct
Frequently Asked Questions
What does CFM stand for and why is it important?
CFM stands for cubic feet per minute, a measure of volumetric airflow. It tells you how much air a fan, blower, or duct system moves in one minute. CFM is the primary metric used to size HVAC equipment, ventilation systems, range hoods, and bathroom exhaust fans. Knowing the correct CFM ensures adequate air circulation for comfort, air quality, and equipment performance without wasting energy.
How do I calculate CFM from room size?
Multiply the room length by width by ceiling height to get the volume in cubic feet. Then multiply by the desired number of air changes per hour and divide by 60 to convert to CFM. The formula is CFM = (Volume x ACH) / 60. For example, a 2,700 cubic foot room needing 6 ACH requires (2,700 x 6) / 60 = 270 CFM. Different room types have different ACH requirements based on their use.
What CFM do I need for a bathroom exhaust fan?
The Home Ventilating Institute recommends 1 CFM per square foot of bathroom floor area, with a minimum of 50 CFM. For bathrooms over 100 square feet, calculate based on the number of fixtures: 50 CFM each for the toilet, shower, and bathtub, and 100 CFM for a jetted tub. A standard 80 square foot bathroom needs an 80 CFM fan. Always vent bathroom fans to the outdoors, never into the attic.
How do I measure actual CFM in a duct?
Use an anemometer to measure air velocity at the duct opening in feet per minute, then multiply by the duct cross-sectional area in square feet. Take velocity readings at multiple points across the duct face and average them for accuracy. A balancing hood or flow capture hood placed over registers gives a direct CFM reading. Digital manometers with pitot tubes are used for measurements inside ductwork.
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Can I use the results for professional or academic purposes?
You may use the results for reference and educational purposes. For professional reports, academic papers, or critical decisions, we recommend verifying outputs against peer-reviewed sources or consulting a qualified expert in the relevant field.