Choose an air purifier by room volume and CADR
Room volume and your target clean-air changes per hour give a particle CADR target. Check the model’s tested rating, replacement filter and usable fan speed before buying.
Back to calculator ↑Measure volume, not floor area alone
Measure length and width in metres or enter known floor area. Multiply by the actual ceiling height: a 4 × 5 m room has 20 m² of floor, but at 2.5 m high it contains 50 m³ of air. A higher ceiling increases the volume even when floor area is unchanged.
A portable purifier serves the air that can circulate through its room. Closed doors, partitions and placement affect that circulation. Measure a genuinely connected space if you expect one unit to serve it, and recognise that the calculator does not model airflow or whole-home air quality.
Calculate the target CADR
The raw target is room volume × chosen air changes per hour, expressed as particle CADR in m³/h. The 4.8 ACH starting value follows AHAM Verifide’s suggested room sizing; you can edit it. For shopping, the tool rounds the raw result up to the next 10 m³/h.
For the 50 m³ example, 50 × 4.8 = 240 m³/h, so the displayed minimum is 240 m³/h. A raw result of 241 would round up to 250, never down. This is a sizing estimate for particle filtration, not a claim that every particle in the room disappears each hour.
Compare model ratings in matching units
Enter the model’s smoke or fine-particle CADR per unit if available, rather than generic fan airflow or advertised room area. The comparison converts cfm with 1 cfm ≈ 1.699 m³/h and multiplies by the selected number of units in the same room.
Two models rated at 120 m³/h each total 240 m³/h arithmetically and meet the example’s raw target. That is a rough shared-room comparison. Placement, partitions, the actual fan speed you tolerate and filter condition all affect useful performance.
Plan filtration and upkeep
Check replacement filter availability and cost, noise at a useful speed, power use and a location with clear intake and outlet paths. Compare the same particle CADR category and units. A quiet low-speed mode may deliver less clean air than the maximum rating.
Filtration supplements source control and suitable ventilation; it does not remove every pollutant. Particle CADR does not rate gas or odour removal. Reduce indoor sources, ventilate when outdoor conditions allow, and avoid devices that intentionally generate ozone.
Common questions
Does equal advertised coverage mean equal CADR?
No. Coverage claims may assume different ACH or ceiling heights; compare particle CADR directly.
Can I add CADR from two units?
The tool adds their ratings for one shared room, but placement and actual airflow can change the result.
Does particle CADR measure odour or gas removal?
No. Investigate gas and odour control separately.
Sources and calculation guidance
Check these against your chosen product’s label and instructions.