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) — and check that figure at the speed you will actually run it, not the number on the box.
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.
Worked through: 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. To check a specific unit instead, divide its CADR by your room volume — if the answer is at least 4 or 5, it fits (see ACH vs CADR).
The ceiling-height trap
Almost every sizing chart you will find online — including the one above — is written for a standard ceiling. That is fine until your room does not have one. You are cleaning a volume, not a floor, so a tall room needs proportionally more clean air for exactly the same footprint.
The same 20 m² room, targeting 5 air changes per hour:
| Ceiling height | Room volume | CADR needed | vs standard |
|---|---|---|---|
| 2.4 m (standard) | 48 m³ | 240 m³/h | — |
| 2.7 m | 54 m³ | 270 m³/h | +13% |
| 3.0 m (Victorian) | 60 m³ | 300 m³/h | +25% |
| 3.5 m (loft / period) | 70 m³ | 350 m³/h | +46% |
A Victorian sitting room or a converted loft can need half as much machine again as the chart suggests. If you live in a period property and a purifier has underwhelmed you, this is the most likely reason — and it is invisible if you only ever compared floor areas.
Size on the speed you will actually use
This is the step that is almost always skipped, and it matters most in the room where it matters most.
A quoted CADR is normally the figure at the highest fan setting. That is the honest maximum, but it is not the setting anyone uses in a bedroom at midnight. If a unit is rated 400 m³/h flat out but delivers perhaps a third of that on its quiet setting, then for overnight use in a bedroom you should be sizing against the quiet figure — not the number on the box.
Two consequences follow. First, a machine with plenty of headroom is worth more than its spec sheet implies, because it reaches your target while still on a low, quiet setting. Second, a machine that only meets your target at full speed will in practice be run below its rating, every night, and quietly underperform the chart.
The practical test when comparing units: ask what the clean-air delivery is at the setting you could sleep next to. If a seller only publishes the maximum, that in itself tells you something.
Four more things that change the answer
- Doors open or closed. Sizing assumes a closed room. Leave the door open and you are effectively cleaning the hallway too, which dilutes the result.
- Where the pollution comes from. A room with a continuous source — a kitchen, a wood burner, a cat — benefits from more capacity than the clearing-time maths alone suggests, because the level you live with while the source is running falls as capacity rises.
- Shape, not just size. An L-shaped or very long room does not mix well; two moderate units at opposite ends beat one large unit in a corner, even at the same total CADR.
- Filter life. Delivered clean air falls as the filter loads, so a unit sized with no margin starts adequate and ends short.
Putting it together
A worked example of the whole process. A 28 m² living room with a 2.7 m ceiling, a cat, and a hay-fever sufferer in the house:
- Volume: 28 × 2.7 = 75.6 m³
- Target: allergies and a continuous pet source, so 6 air changes, not 4–5
- CADR needed: 75.6 × 6 ≈ 454 m³/h
- Check at quiet speed: pick a unit that reaches ~454 m³/h without going to maximum
The quick chart would have suggested 400–550 m³/h for this room, so the answer lands in the same range — but the reasoning tells you where in that range to sit, and that is usually the part people get wrong.
Free calculator: your exact CADR
Enter your room to get the CADR to aim for — or paste a purifier's CADR to check whether it's big enough:
Live UK air quality
On high-pollution days indoor levels climb too, so a well-sized unit matters most. Here's the current reading:
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.
Looking for a unit that does this well? The Luggable XL is #1 of 247 on the US ENERGY STAR Most Efficient list, lab-tested by Intertek, and uses standard 3M MERV-13 filters.
FAQ
Is it bad to get a purifier that's too big? No — an oversized unit reaches the same result at a lower fan speed, which is quieter and draws less power. 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, and expect to want either a high-CADR unit or two placed apart.
Do I need a bigger unit for a loft conversion? Usually yes — see the ceiling-height table. Sloped ceilings make it slightly kinder than a flat 3.5 m, but still well above the standard chart.
Does a bigger unit cost more to run? Not necessarily. Running a large unit gently can draw less power than a small one at full speed for the same clean-air delivery, because fan power rises steeply with speed.
How do I size for a nursery? Size as normal, then judge on noise — in a nursery the setting you will actually use is the lowest one, so headroom matters more than the headline figure.