How to size an air purifier for your room
Updated · By the purifierplan.com team
Multiply your room's volume by the number of air changes an hour you want. That gives the clean air output the purifier has to deliver. Then allow extra if you'll run it on a quieter setting.
Clean air needed (m³/h) = length × width × ceiling height × air changes per hour. A typical HDB common bedroom (about 2.7 × 3.2 m with a 2.6 m ceiling) needs about 112 m³/h for 5 air changes an hour. A purifier that has to do that on medium speed should have a rated CADR of about 160 m³/h.
Step 1: Measure the room
Measure length and width in metres and multiply them for the floor area. Multiply by the ceiling height to get the volume. Many HDB flats have ceilings around 2.6 m, but measure yours, because a taller room holds more air.
If rooms are open to each other, such as a living room, dining area and an open kitchen, treat them as one space. Air moves freely between them, so a purifier in one part is also cleaning the rest. A bedroom with its door shut is its own space.
Step 2: Choose how many air changes an hour
An air change means the purifier has passed the room's whole volume of air through its filter once. More changes an hour means the room's air is filtered faster. During haze, outdoor smoke keeps leaking in through gaps around windows and doors, so the purifier has to keep up with it.
- 3 an hour (basic): a noticeable improvement in a well-sealed room.
- 5 an hour (standard): the default in our planner.
- 7 an hour (extra margin): for very hazy days, or if you'd simply like more filtration.
For comparison, AHAM's rule of thumb (a smoke CADR in cfm of at least two-thirds of the floor area in square feet) works out to about 5 air changes an hour in a room with an 8-foot ceiling. Its advice for wildfire smoke, a smoke CADR equal to the floor area, works out to about 7.5 (AHAM).
Step 3: Allow for the speed you'll actually use
A purifier's CADR is measured at top speed, which is usually too loud to sleep with. If you plan to run it on medium, it will deliver less clean air. Our planner assumes medium gives about 70% of the rated output, so it divides the clean air needed by 0.7. That's an assumption, not a test result, and the real figure varies by model.
Step 4: Read the CADR on the box carefully
- Units: US figures are in cubic feet per minute (cfm). 1 cfm ≈ 1.699 m³/h.
- Smoke, dust or pollen: use the smoke figure for haze.
- Who measured it: some figures are independently verified under the AHAM programme, and others are the brand's own figure under a different method, such as China's GB/T 18801. The methods differ, so the numbers aren't directly comparable. We label each one "Tested" or "Brand's figure".
Typical HDB rooms
| Room | Floor | Needed at 5/h | Rated CADR, 5/h at medium | Rated CADR, 7/h at medium |
|---|---|---|---|---|
| 2-room Flexi (36 m²) bedroom | 7.8 m² | 101 m³/h | 145 m³/h | 203 m³/h |
| 4-room common bedroom | 8.6 m² | 112 m³/h | 160 m³/h | 225 m³/h |
| 5-room common bedroom | 9.5 m² | 124 m³/h | 177 m³/h | 248 m³/h |
| 3-room master bedroom | 12.5 m² | 163 m³/h | 233 m³/h | 326 m³/h |
| 4-room main bedroom | 12.9 m² | 168 m³/h | 240 m³/h | 335 m³/h |
| 4-room living / dining | 24.6 m² | 320 m³/h | 458 m³/h | 641 m³/h |
| 5-room living / dining | 29.8 m² | 387 m³/h | 553 m³/h | 774 m³/h |
The living-room figures are for the living and dining area alone. If your kitchen and corridor are open to it, add their volume too. In our worked 4-room example, the whole open area comes to 121 m³.
One big purifier or two small ones?
For a large open-plan space, two smaller units placed apart spread clean air more evenly than one unit in a corner, and each can run slower. One large unit is simpler and often cheaper to run. Our planner compares both, including filter and electricity costs, and shows which corners get too little clean air.
Size every room at once. The planner works this out for your whole home, shows where clean air reaches on a floor plan, and lets you choose how many purifiers to buy.
Plan my homeSources
- AHAM, AHAM's air filtration standards (2/3 rule, wildfire smoke advice, 8-foot ceiling). Checked 15 October 2026.
- Air changes per hour and the 0.7 medium-speed factor are the planner's assumptions, explained on How we rank.
Questions people ask
What CADR do I need for a 10 m² bedroom?
With a 2.6 m ceiling the room holds 26 m³ of air. For 5 air changes an hour it needs 130 m³/h of clean air. If you want the purifier to manage that at medium speed, look for a rated CADR of about 186 m³/h (110 cfm).
Is a bigger air purifier better?
A bigger purifier cleans the room faster and can do the job at a lower, quieter speed. It costs more to buy and its filters usually cost more. Size for the room and the target you want, then pick by cost and noise.
What is a good number of air changes per hour during haze?
There's no official Singapore figure for homes. Our planner offers 3 (basic), 5 (standard) and 7 (extra margin). AHAM's general rule of thumb works out to about 5 in an 8-foot room, and its wildfire-smoke advice works out to about 7.5.
Should I use the smoke, dust or pollen CADR?
Use the smoke CADR for haze. Haze is mostly fine particles, and smoke particles are the smallest of the three test particles, so the smoke figure is the closest match.
How do I convert cfm to m³/h?
Multiply cfm by 1.699. For example, 200 cfm is about 340 m³/h.