Air purifier sizing
Air Purifier CADR and Room Size: Use the Airflow Math
Square foot claims are easy to market. CADR lets you compare how much clean air the machine can actually deliver.
The quick answer
What should you do first?
For particle filtration, compare the purifier's CADR with the size and volume of the room. EPA's current sizing table uses about 65 CFM of CADR for each 100 square feet with an 8 foot ceiling. Another useful calculation is equivalent air changes per hour: multiply CADR in CFM by 60 and divide by room volume in cubic feet. Higher ceilings increase room volume and reduce the air change rate from the same purifier.
Start with these checks
- 01
Find the particle CADR
Use a recognized clean air delivery rate when available. CADR is a measure of clean particle filtered airflow. It does not by itself rate gas or VOC removal performance.
- 02
Calculate room volume
Multiply room length by width by ceiling height. A 20 by 15 foot room with an 8 foot ceiling contains 2,400 cubic feet of air.
- 03
Calculate equivalent air changes per hour
Use eACH = CADR in CFM × 60 ÷ room volume in cubic feet. A 235 CFM purifier in a 300 square foot room with an 8 foot ceiling provides about 5.9 equivalent air changes per hour at the rated airflow.
- 04
Compare several room sizes instead of one marketing number
The same 235 CFM is about 3.5 eACH in 500 square feet with an 8 foot ceiling and about 1.8 eACH in 1,000 square feet. This makes the tradeoff between coverage and cleaning rate visible.
- 05
Account for real operating conditions
CADR is typically associated with a high fan setting. Lower fan speeds, filter loading, room layout, door position, pollutant sources, and placement affect real performance.
EPA's table provides a useful reality check
EPA lists minimum CADR estimates of 65 CFM for 100 square feet, 130 CFM for 200, 195 CFM for 300, 260 CFM for 400, 325 CFM for 500, and 390 CFM for 600, based on an 8 foot ceiling. The agency also notes that higher ceilings call for a larger unit.
That table is not the only possible sizing method, but it is a useful neutral benchmark when manufacturer coverage claims use different test assumptions.
What a 400 m³/h CADR means in ordinary rooms
Dreame publishes a 400 m³/h CADR for the AirPursue PM20. That converts to about 235 CFM. Using the EPA table's approximate relationship, 235 CFM corresponds to roughly 360 square feet at an 8 foot ceiling. Using an equivalent air change calculation, it is about 5.9 eACH in 300 square feet, 3.5 eACH in 500 square feet, and 1.8 eACH in 1,000 square feet at an 8 foot ceiling.
Dreame also publishes much larger coverage figures. Those numbers can use different time windows and performance definitions, so they should not be interpreted as the same thing as maintaining a high equivalent air change rate across the entire stated area. For comparison, 235 CFM in 1,690 square feet with a 9 foot ceiling is about 0.93 eACH from the purifier alone.
CADR does not settle every air quality question
EPA explains that CADR is a particle filtration metric. Gas and VOC removal depend on different filter media and do not have the same widely used consumer performance rating system. A purifier can therefore have a strong particle CADR without that number proving how quickly it removes formaldehyde, odors, or other gases.
Clear answers
Common questions
What is CADR?
Clean air delivery rate represents how much particle filtered air an air cleaner can deliver. Higher CADR generally supports a larger room or a higher cleaning rate in the same room.
How do I calculate air changes per hour from CADR?
Convert CADR to cubic feet per minute if needed, multiply by 60, then divide by the room volume in cubic feet. The result is an equivalent air change rate attributable to the purifier at that airflow.
Does a 1,700 square foot coverage claim mean the purifier cleans that space five times per hour?
Not necessarily. Coverage claims can be based on different time periods and assumptions. Use the CADR and room volume to calculate the equivalent air change rate you would actually expect from the rated clean airflow.
Related troubleshooting
- 01
Central Air Conditioner Not Cooling? Check Airflow Before Refrigerant
Poor cooling can come from control settings, restricted airflow, dirty heat-transfer surfaces, outdoor-unit failure, or refrigerant problems. Start with what you can verify safely.
- 02
How to Stop Drafts From Windows Without Replacing Them
Find the leak path first. A draft at a moving sash needs a different repair than a gap behind stationary trim.
- 03
How to Prevent Bathroom Mold by Fixing the Moisture Problem
Recurring mold is usually a moisture problem first. Cleaning matters, but the room must also dry reliably after showers.
Evidence
Sources and method
This guide combines a maintenance troubleshooting approach with current technical guidance from the sources below. Safety rules, manufacturer instructions, and applicable codes take priority over general troubleshooting guidance.