What size air purifier do I need?
In one line: match the purifier's CADR to your room so it delivers about 4–5 air changes per hour (5–6+ for allergies or smoke). Bigger room = higher CADR needed. When in doubt, size up and run it gently.
The quick room-size guide
For a standard 2.4 m ceiling, aiming for ~4–5 air changes per hour:
| Room | Floor area | CADR to aim for |
|---|---|---|
| Small bedroom / office / nursery | up to 15 m² (≈160 sq ft) | 150+ m³/h |
| Double bedroom / lounge | 15–25 m² (≈160–270 sq ft) | 250–350 m³/h |
| Large living room | 25–40 m² (≈270–430 sq ft) | 400–550 m³/h |
| Open-plan / kitchen-diner | 40–60 m² (≈430–650 sq ft) | 550–700 m³/h |
The formula, if you want to be exact
CADR needed = room volume (m³) × target air changes. Room volume = floor area × ceiling height.
Example: a 20 m² lounge with a 2.4 m ceiling is 48 m³. For 5 air changes per hour you need 48 × 5 = 240 m³/h CADR. To check a specific unit, divide its CADR by your room volume — if it gives at least 4–5, it fits (see ACH vs CADR).
Practical tips
- Size up, run low. A bigger unit at a gentle speed is quieter and cleans faster than a small one flat-out.
- One unit per closed room. Air won't clean through walls; a big open-plan space may need two.
- Go higher for allergies/smoke — aim for 5–6+ air changes so spikes clear fast.
- Mind the ceiling height — tall or loft rooms need more CADR than the floor area alone suggests.
Next step
Compare units by measured CADR at Best Air Purifier UK, and check the running cost of your shortlist with the running-cost calculator.
FAQ
Is it bad to get a purifier that's too big? No — an oversized unit just cleans faster and can run quietly. Too small is the real problem: it never keeps up.
What if my room is open-plan? Treat the whole connected volume as the room; you'll likely need a high-CADR unit or two units.