Air Purifier Size Calculator
Find the right CADR and purifier size based on your room dimensions and air changes per hour (ACH).
How to Choose the Right Air Purifier Size
Selecting the correct air purifier for your space is vital for achieving optimal air quality. The key metric is the Clean Air Delivery Rate (CADR), which indicates how quickly an air purifier can remove particles from the air. Use this calculator to determine the ideal CADR rating based on your room size and how often you want the air cleaned (measured in Air Changes per Hour or ACH).
What Is CADR?
CADR stands for Clean Air Delivery Rate and is measured in cubic feet per minute (CFM). It tells you how much filtered air the purifier can deliver. A higher CADR means faster and more effective filtration.
What Is ACH?
ACH (Air Changes per Hour) refers to how many times the entire air volume of a room is filtered in one hour. For example:
- 2 ACH – Suitable for minimal air quality issues
- 4 ACH – Good for most households
- 6 ACH – Ideal for allergy sufferers
- 8+ ACH – Necessary for smokers, pets, or industrial settings
CADR Calculation Formula
CADR = (Room Volume × ACH) / 60
Where:
- Room volume = Length × Width × Height (in cubic feet)
- ACH = Number of air changes per hour
- 60 = Number of minutes per hour
Example Calculation
Room: 15 × 12 × 8 ft → 1,440 ft³
Desired ACH: 6
CADR = (1,440 × 6) / 60 = 144 CFM
So you would need a purifier with at least a 144 CADR rating.
CADR Ratings by Room Size
| Room Size (sq ft) | Room Volume (ft³) | ACH 4 | ACH 6 |
|---|---|---|---|
| 150 | 1,200 | 80 CFM | 120 CFM |
| 300 | 2,400 | 160 CFM | 240 CFM |
| 500 | 4,000 | 267 CFM | 400 CFM |
Other Sizing Tips
- Look for CADR ratings specifically for smoke, pollen, and dust
- Avoid oversized purifiers that waste energy
- Don’t forget about filter replacement cost and frequency
Common Questions
Q: Can I use a purifier in a larger room than rated?
A: It will work, but less effectively. The air may not be filtered fast enough to reduce allergens or odors efficiently.
Q: What CADR rating is good for asthma or allergies?
A: Look for a purifier with 6 ACH or more, and a CADR at least 2/3 of the room’s square footage.
Q: Is higher ACH always better?
A: More ACH equals faster purification, but may create more noise or drafts. Match it to your needs.
Conclusion
Choosing the right air purifier depends on your room size, air quality goals, and sensitivity to allergens. This calculator simplifies the math by giving you an accurate CADR recommendation based on your chosen ACH level. Use it to match your space with the right unit and breathe cleaner, healthier air.