CFM Calculator
Calculate cubic feet per minute (CFM) for ventilation, HVAC, and exhaust fan sizing. Enter room dimensions and air changes per hour for accurate results.
Formula & How It Works
CFM = (Room Volume × Air Changes per Hour) / 60
Room volume is length × width × ceiling height in cubic feet. Multiply by the desired air changes per hour (ACH) and divide by 60 to convert to cubic feet per minute. The ACH requirement varies by room type - bathrooms need 8 ACH, kitchens 6 ACH, and living areas 4 ACH.
Recommended Air Changes Per Hour (ACH)
| Room Type | Recommended ACH | Air Exchange Every |
|---|---|---|
| Attic | 2-4 ACH | 15-30 min |
| Basement | 3 ACH | 20 min |
| Bedroom | 2-4 ACH | 15-30 min |
| Living Room | 4 ACH | 15 min |
| Kitchen (residential) | 6 ACH | 10 min |
| Bathroom | 8 ACH | 7.5 min |
| Workshop / Garage | 10 ACH | 6 min |
| Commercial Kitchen | 15-20 ACH | 3-4 min |
| Industrial / Lab | 20+ ACH | 3 min |
CFM Sizing Tips
- Bathroom fans: HVI recommends 1 CFM per sq ft for bathrooms under 100 sq ft
- Range hoods: Need 100-400 CFM for residential, 500+ CFM for commercial kitchens
- Makeup air: Exhaust fans over 400 CFM may require makeup air to avoid backdrafting
- Static pressure: Ductwork, filters, and bends reduce actual CFM - size 20% larger
- ASHRAE standards: Follow ASHRAE 62.2 for residential ventilation requirements
Frequently Asked Questions
How do I calculate CFM for a room?
Multiply the room length × width × ceiling height to get the volume in cubic feet. Multiply by the desired air changes per hour (ACH) and divide by 60. For example, a 20×15×8 room (2,400 cu ft) at 4 ACH needs (2,400 × 4) / 60 = 160 CFM.
How many CFM do I need for a bathroom?
For bathrooms under 100 sq ft, the HVI recommends 1 CFM per square foot of floor area. A 50 sq ft bathroom needs a 50 CFM fan. For bathrooms over 100 sq ft, calculate based on 8 air changes per hour using the room volume method.
How many CFM do I need for a kitchen range hood?
Residential kitchens typically need 100-300 CFM for electric ranges and 250-400 CFM for gas ranges. For every 10,000 BTU of gas burner output, add 100 CFM. Commercial kitchens need 500-2,000+ CFM depending on cooking equipment. If using over 400 CFM, install makeup air to prevent backdrafting.
What is ACH (air changes per hour)?
ACH measures how many times the entire volume of air in a room is replaced in one hour. A room with 4 ACH has its air completely replaced every 15 minutes. Higher ACH values mean more ventilation, which improves air quality but increases energy costs for heating and cooling the incoming air.
Why does my exhaust fan move less CFM than rated?
Manufacturers rate fans at zero static pressure (no ductwork). In real installations, duct length, bends, filters, and exterior vents create resistance that reduces actual CFM by 20-40%. To compensate, select a fan rated 25-50% higher than your calculated requirement, or use smooth rigid ductwork.