Air Purifier CADR & Room Size Calculator
Enter the room size, ceiling height, ACH target, and the purifier CADR you are considering. The calculator shows the CADR required, the ACH one unit can deliver, and whether you need more than one unit.
Required CADR120
- Room volume
- 1,440
- Effective purifier CADR
- 100
- ACH from one purifier
- 4.17
- Purifiers required
- 2
- Maximum supported floor area
- 150
- Candidate fit
- 20 CFM short; use 2 identical units
Uses the product's smoke / PM2.5 CADR, not raw fan airflow.
Published CADR usually reflects a new filter at the tested speed.
How to use this calculator
- Measure the full connected space the purifier will serve, including open rooms that share air.
- Choose the target air changes per hour for the particle concern you care about.
- Enter the matching smoke, dust, or pollen CADR from the product label.
- Adjust usable CADR only when you have a published lower-speed CADR or a deliberate derating assumption.
How the CADR requirement is calculated
CADR, or clean air delivery rate, is the amount of filtered air a purifier supplies for a specific particle type, reported in cubic feet per minute. A room-size rating is a shortcut built from that airflow, the room volume, and the number of air changes per hour you want.
This calculator first finds room volume, then converts the target air changes per hour into the CADR needed:
room volume = length × width × ceiling height
required CADR = room volume × target ACH ÷ 60
The required CADR is rounded up to the next whole CFM because buying a purifier with less clean-air delivery misses the target. The purifier you enter is adjusted for the speed you expect to use:
effective CADR = listed CADR × usable CADR percent ÷ 100
The achieved ACH from one unit is effective CADR × 60 ÷ room volume. If one unit falls short, the required unit count is the required CADR divided by effective CADR, rounded up.
Why the AHAM shortcuts work
AHAM's common two-thirds room-area rule assumes an 8-foot ceiling and about five air changes per hour. With an 8-foot ceiling, area × 8 × 5 ÷ 60 equals about area × 0.67. AHAM's wildfire-smoke guidance uses a room area roughly equal to smoke CADR, which corresponds to about 7.5 ACH in the same 8-foot room.
What changes the result most
Floor area, ceiling height, and ACH target move the required CADR directly. A room with a 10-foot ceiling needs 25% more CADR than the same floor area with an 8-foot ceiling. Running a purifier below its tested speed also matters: a listed 200 CFM unit treated as 60% usable behaves like a 120 CFM unit for this calculation.
What this calculator leaves out
The result is a sizing estimate for particles, not a prediction of measured indoor air quality. It does not model open windows, outdoor-air leakage, active particle sources, poor placement, blocked intake or exhaust, loaded filters, gases, odors, carbon monoxide, or volatile organic compounds. Use the pollutant-specific CADR from the product label; raw fan CFM is not a substitute for tested clean-air delivery.
Worked example
For a 15-foot by 12-foot room with an 8-foot ceiling, the room volume is 1,440 cubic feet. At a target of 5 ACH, the required CADR is 1,440 × 5 ÷ 60 = 120 CFM.
If the purifier lists a 100 CFM smoke CADR and you plan to use its full tested output, the effective CADR is 100 CFM. One unit delivers 100 × 60 ÷ 1,440 = 4.17 ACH, supports about 150 square feet at 5 ACH and an 8-foot ceiling, and is 20 CFM short. The calculator therefore recommends two identical units for that target.
Common questions
What CADR do I need for my room size?
Multiply the room length, width, and ceiling height to get cubic feet, then multiply by your target ACH and divide by 60. For a normal 8-foot ceiling at 5 ACH, this is about two-thirds of the floor area in CFM. Taller rooms or higher ACH targets need more CADR.
Why are smoke, dust, and pollen CADR ratings different?
The tests use different particle sizes, and a purifier may remove them at different rates. Smoke CADR is usually the relevant label value for fine particles such as PM2.5 and wildfire smoke. Use the CADR for the particle concern you selected rather than the highest number on the box.
How does ceiling height change the required CADR?
CADR is sized against room volume, not just floor area. Package square-foot ratings commonly assume an 8-foot ceiling, so a 10-foot ceiling needs 25% more CADR for the same floor area and ACH target. The calculator accounts for that by using cubic feet.
How many air changes per hour should I target?
Five ACH matches AHAM's approximate two-thirds sizing rule for an 8-foot room and is a common general sizing target. About 7.5 ACH matches AHAM's wildfire-smoke room-area guidance in an 8-foot room. Higher targets can be useful for stronger particle control, but they often require more than one consumer purifier.
Can I combine two smaller air purifiers?
Yes, CADR is additive when both units serve the same connected space and have unobstructed airflow. Two identical 100 CFM units provide about 200 CFM of effective CADR at the tested speed. Placement still matters because dead zones and blocked airflow can reduce real-world performance.
Are advertised square-foot ratings based on maximum fan speed?
They often are based on tested CADR at a high or maximum speed and on an assumed ceiling height. If you plan to run the purifier on a quieter speed, use a manufacturer-published lower-speed CADR when available. The usable-CADR percentage lets you test that scenario without pretending the listed maximum CADR always applies.