How to Use This Calculator
This tool helps you determine the correct air purifier size for any room in your home based on volume and desired air circulation rate.
- Measure your room: Enter the length, width, and ceiling height in feet
- Select air changes per hour (ACH): Choose based on your air quality needs and sensitivity
- Review your results: The calculator shows the minimum CADR (Clean Air Delivery Rate) you need
- Shop accordingly: Look for air purifiers with CADR ratings that meet or exceed the calculated value
Understanding CADR
CADR (Clean Air Delivery Rate) measures how much filtered air an air purifier delivers, expressed in cubic feet per minute (CFM). A higher CADR means the unit can clean air faster and handle larger spaces.
Understanding ACH (Air Changes Per Hour)
ACH indicates how many times per hour the entire volume of air in a room passes through the purifier. More air changes mean cleaner air:
- 2 ACH: Basic air circulation for general rooms
- 4-5 ACH: Recommended for most residential use, effective for allergies
- 6 ACH: Good for homes with pets or cooking odors
- 8+ ACH: Needed for smoke removal, severe allergies, or high pollution areas
Example Calculation:
A 15' × 12' bedroom with 8' ceilings = 1,440 cubic feet
At 5 ACH: (1,440 × 5) ÷ 60 minutes = 120 CFM minimum CADR
You should look for an air purifier rated for at least 120 CFM or a room size of 180 square feet.
Common Mistakes to Avoid
- Ignoring ceiling height: Standard calculators assume 8-foot ceilings. Tall ceilings require more powerful units
- Buying based on square footage alone: Volume matters more than floor area for air purification
- Not matching CADR to all pollutants: Check that smoke, dust, and pollen CADR values all meet your needs
- Forgetting about doorways: Open floor plans need higher CADR or multiple units
- Undersizing for specific needs: Smoke, pets, and strong allergens require higher ACH rates
Assumptions Used in This Calculator
- Room is a single enclosed space (adjust for open floor plans)
- Standard residential construction with typical air leakage
- CADR is calculated using the formula: (Volume × ACH) ÷ 60
- Recommended room coverage is typically 1.5× the actual square footage for effective cleaning
- Filter efficiency and maintenance are assumed to be optimal
This calculator provides guidance for residential air purifier selection. For commercial, medical, or industrial applications, consult HVAC professionals.
Frequently Asked Questions
What if my room has an open doorway to another room?
For open floor plans or connected rooms, calculate the combined volume of all connected spaces. Alternatively, use multiple air purifiers in different zones.
Is a higher CADR always better?
Higher CADR provides faster air cleaning, but oversized units may be noisier and more expensive to run. Match CADR to your room size and needs for the best balance of performance and efficiency.
How often should I run my air purifier?
For best results, run your air purifier continuously, especially during allergy season or when air quality is poor. Most modern units are energy-efficient and designed for 24/7 operation.
What if the manufacturer lists room size instead of CADR?
Manufacturers often list maximum room size assuming 8-foot ceilings and 2 ACH. For allergy or asthma relief, choose a unit rated for 2-3 times your actual room size to achieve higher ACH rates.
Do I need different CADR for smoke vs. dust?
CADR is measured separately for smoke, dust, and pollen because filter efficiency varies by particle size. For comprehensive protection, ensure all three CADR values meet your calculated minimum.
Can I use one large air purifier for my whole house?
Air purifiers work best in enclosed spaces. For whole-house coverage, either use multiple units or install a whole-house air purification system in your HVAC ductwork.