What is CFM?
CFM (Cubic Feet per Minute) is the standard unit of measurement for airflow volume in HVAC and ventilation systems in the United States. It measures the volume of air that moves past a given point in one minute. Understanding CFM is essential for properly sizing exhaust fans, supply ducts, air handlers, and ventilation systems in construction projects.
Every occupied space requires a certain amount of fresh air exchange to maintain indoor air quality. Building codes specify minimum ventilation rates in CFM based on the room type, occupancy, and activities conducted. Insufficient ventilation leads to poor air quality, moisture problems, and health issues.
CFM Formula
CFM Requirements by Application
| Application | Recommended ACH | Typical CFM Range |
|---|---|---|
| Bathroom (residential) | 8 | 50-110 CFM |
| Kitchen range hood | 10-15 | 100-600 CFM |
| Laundry room | 6-8 | 100-200 CFM |
| Office space | 4-6 | Varies by occupancy |
| Restaurant kitchen | 15-20 | 1,000-5,000 CFM |
| Garage (residential) | 4-6 | 200-500 CFM |
Unit Conversions
- 1 CFM = 0.4719 liters per second (L/s)
- 1 CFM = 1.699 cubic meters per hour (m³/hr)
- 1 CFM = 28.317 liters per minute (L/min)
- 1 L/s = 2.119 CFM
Frequently Asked Questions
What CFM fan do I need for a bathroom?
The HVI (Home Ventilating Institute) recommends 1 CFM per square foot of bathroom floor area, with a minimum of 50 CFM. For a standard 80 sq ft bathroom, an 80 CFM fan is recommended. For bathrooms with jetted tubs or steam showers, increase to 1.5 CFM per square foot.
How do I calculate CFM for a kitchen range hood?
For a wall-mounted hood, multiply the cooking surface width (inches) by 1.5 to get minimum CFM. For island hoods, multiply by 2. For example, a 30-inch range against a wall needs at least 45 CFM, but 100+ CFM is recommended for gas cooktops and 300+ CFM for professional ranges.
Does ductwork size affect CFM?
Yes. Undersized ductwork restricts airflow and reduces effective CFM. For example, a 100 CFM fan needs at minimum a 4-inch round duct. Longer duct runs and more elbows increase resistance, requiring a more powerful fan to achieve the desired CFM at the discharge point.