How to use this calculator
Start by entering your room's length and width in feet, then confirm the ceiling height (most rooms are 8 feet). Select the air quality concern that matters most to you so the calculator can tailor its CADR and filter recommendations accordingly.
Choose how many air changes per hour (ACH) you want. Standard (2 ACH) works for general maintenance, recommended (4 ACH) is ideal for allergy sufferers, and high (6 ACH) is best for smoke, strong odors, or anyone with respiratory sensitivities.
If you plan to cover multiple rooms with a single purifier, increase the room count. The calculator will scale its recommendations for the total area, though separate units for each room will always outperform a single large unit.
Understanding the two thirds rule for CADR
The Association of Home Appliance Manufacturers (AHAM) recommends that an air purifier's smoke CADR be at least two thirds of the room's floor area in square feet. For example, a 200 square foot room needs a minimum smoke CADR of about 133. Dust and pollen CADR requirements follow the same proportional rule.
This guideline ensures the purifier can cycle all the air in the room frequently enough to maintain good air quality. Going above the minimum CADR allows the purifier to run on lower, quieter fan speeds while still achieving the same number of air changes.
Frequently asked questions
What CADR rating do I need for my room?
A good rule of thumb is that the CADR should be at least two thirds of your room's square footage. For a 200 sq ft bedroom, aim for a CADR of at least 133. For allergy or smoke concerns, increase that to match or exceed the full square footage.
How many air changes per hour should an air purifier provide?
For general air quality, 2 air changes per hour (ACH) is adequate. For allergy relief, 4 ACH is recommended. For smoke, odors, or severe allergies, aim for 6 ACH. Higher ACH means the air passes through the filter more frequently, removing more particles.
What is the difference between HEPA and activated carbon filters?
HEPA filters capture 99.97% of particles 0.3 microns and larger, including dust, pollen, mold spores, and pet dander. Activated carbon filters absorb gases, odors, and volatile organic compounds (VOCs). Many purifiers combine both filter types for comprehensive air cleaning.
How much does it cost to run an air purifier all year?
Most air purifiers use between 30 and 100 watts, costing roughly $25 to $90 per year in electricity when run continuously. Filter replacements add $40 to $120 annually depending on the model. Total annual operating cost for a mid size unit is typically $80 to $150.