Matching an air purifier to your room size
In this guide
Why CADR matters more than the brand
CADR (Clean Air Delivery Rate) is the volume of clean air the unit puts out per hour, measured in cubic metres per hour. It tells you plainly how fast the machine cleans.
A rough calculation for a bedroom:
- Measure the room in square metres, then multiply by ceiling height (typically 2.5 m) to get the volume in m³
- Pick a CADR of at least three times that volume
Example: a 20 m² room × 2.5 m = 50 m³, so you want a CADR of roughly 150–200 m³/h or higher.
Watch out: advertised CADR figures are usually measured at the highest fan speed,
which is loud. On the quiet night mode, real CADR is far lower — check the quiet-mode figure too.
Which filter grade, and how often to replace
| Filter type | What it captures | Service life |
|---|---|---|
| Pre-filter | Large dust, pet hair | Washable, reusable |
| HEPA H13 | PM2.5, pollen, allergens | 6–12 months |
| Activated carbon | Odours, VOCs | 3–12 months |
| Combined HEPA + carbon | Both | 6–12 months (replaced together) |
HEPA H13 captures 99.95% of particles at 0.3 microns, which is the grade to look for if PM2.5 is your concern. H11 or anything sold as "HEPA-like" performs noticeably worse.
Noise and the bedroom problem
The acceptable level for sleeping is below 30 decibels. If a spec sheet does not state the quiet-mode noise figure, treat that as a warning sign — manufacturers who genuinely achieve it usually state it clearly.
Working out the yearly running cost
This is the part most people skip:
| Item | Approximate yearly cost |
|---|---|
| Combined filter (once a year) | Varies by model — check the replacement filter price before buying |
| Electricity (8 h per night) | An efficient model runs roughly 100–200 baht per year |
Tip: before you buy, search for the replacement filter price for that specific model.
If you cannot find one, or it costs close to half the unit price, the running cost is high.
Pre-purchase checklist
- ☐ Room volume calculated, and CADR is at least 3× that volume
- ☐ True HEPA H13 filter (not H11 or "HEPA-like")
- ☐ Carbon filter included if you need odour removal
- ☐ Quiet-mode noise level stated explicitly
- ☐ Replacement filter available and reasonably priced
- ☐ Real-time PM2.5 sensor if you want to see readings
- ☐ Local service centre or authorised distributor
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We may earn a commission on qualifying purchases — the price you pay does not increase,
and we have not physically tested every product.