For a room measuring 4 by 3 metres with a 2.7-metre ceiling, the calculator gives about 156 m³/h at 4.8 air changes per hour. The calculation is room volume multiplied by the selected air-change rate. Use it to compare specifications, not as a guarantee of air quality.
Calculate CADR for your room
Enter the room dimensions and the number of air changes per hour you want to compare. Changing units converts the dimensions already entered.
Measure the room, including ceiling height
For a bedroom, measure the bedroom itself. For an open-plan living and dining area, include both spaces if you want the purifier to serve them together. A unit in the lounge may not move enough cleaned air down a corridor into separate bedrooms.
If the floor plan is L-shaped, split it into two rectangles and calculate each area. With the same ceiling height throughout, add the areas and multiply by that height. For example, sections of 12 m² and 6 m² with a 2.7 m ceiling give 48.6 m³. At 4.8 air changes per hour, that works out to 233.28 m³/h, rounded up to 234. If the ceiling heights differ, calculate each section's volume separately before adding them.
Ceiling height makes a difference: a 20 m² room contains 48 m³ of air with a 2.4 m ceiling, or 60 m³ with a 3 m ceiling. The taller room needs 25% more CADR for the same air-change rate.
What do CADR and ACH mean?
CADR stands for clean air delivery rate. It describes how quickly a purifier supplies air cleaned of particles. AHAM gives separate ratings for smoke, dust and pollen, so compare the same type of rating when looking at two models. A fan-airflow figure alone does not tell you how much particle-free air the purifier delivers.
ACH means air changes per hour. In this calculator, it describes the equivalent room volume of cleaned air supplied each hour. It does not mean the purifier brings in fresh outdoor air.
The default is 4.8 ACH because AHAM uses that rate for its suggested room-size rating. Leave it at 4.8 for this comparison, or change it if you are working from another specified rate. It is a sizing assumption, not a guarantee that every pollutant will be removed.
Worked examples
| Room dimensions | Volume | CADR at 4.8 ACH |
|---|---|---|
| 3 × 3 × 2.7 m | 24.3 m³ | 116.64 m³/h; round up to 117 |
| 4 × 3 × 2.7 m | 32.4 m³ | 155.52 m³/h; round up to 156 |
| 5 × 4 × 2.8 m | 56 m³ | 268.8 m³/h; round up to 269 |
These are calculated examples, not measurements from specific homes. Round upward when shortlisting, then check whether the product delivers that amount at a fan speed you will actually use.
Why two room-coverage claims may differ
A listing that says “covers 60 m²” leaves out part of the picture. Look for the ceiling height, air-change rate and fan setting behind that claim. A purifier advertised for a large area at a low air-change rate may deliver less cleaned air than one advertised for a smaller area at a higher rate.
Check the units too. A CADR of 100 CFM is about 170 m³/h, not 100 m³/h. The calculator shows both units so you can compare specifications without converting them yourself.
For overnight use, check the CADR at the speed you would actually leave running. Maximum CADR and the quietest advertised noise level may refer to different settings. The bedroom guide explains what to check for sleeping comfort; the buying guide covers filters, features and ongoing costs.
Before choosing a purifier
Keep a note of your room measurements, selected ACH and calculated CADR when comparing models. Check the product manual for clearance around the intake and outlet, and make sure replacement filters are available. The placement guide helps you choose a spot with clear airflow.
A correctly sized purifier is only part of managing indoor air. Blocked intakes, dirty filters and ongoing sources of particles affect results. EPA guidance recommends reducing pollution sources and ventilating with clean outdoor air alongside filtration.
For haze, follow current local advice and the haze operating guide. The calculator's default is a general room-sizing comparison, not an emergency smoke recommendation.
Frequently asked questions
Can one purifier clean the whole condo?
A portable purifier is intended for one room or area. Adding up your condo’s total floor area does not show whether cleaned air will reach separate bedrooms. Size it for the room or connected space where you will use it.
Can I enter measurements in feet?
Yes. Select Feet to convert the dimensions already entered, or type your measurements after selecting the unit. The result shows CADR in both m³/h and CFM.
Sources and further reading
Sources checked on 11 September 2026.
