Short answer
Multiply your room’s floor area in square metres by 12, and look for at least that CADR in m³/h. A 14 m² bedroom needs about 170 m³/h, a 30 m² living room about 360 and a 48 m² open-plan room about 580. That rule gives five air changes an hour under a 2.4 m ceiling, more than most makers assume, which is why their room sizes look bigger.
The short answer
- CADR decides the room size. It is the clean air a purifier delivers on top speed; divide it by 12 for the largest room it suits at five air changes an hour.
- Don’t trust the room size on the box. Makers assume anything from under two to about five air changes an hour, so the same CADR can be sold for a room two or three times bigger.
- Buy some headroom. CADR is measured on top speed only. A purifier sized above your room can clean it on a quieter setting, which matters most in a bedroom.
Work out the CADR your room needs
What it is: CADR, the clean air delivery rate, is how fast a purifier lowers the particles in a test room, given as the volume of clean air it adds each hour (m³/h) or each minute (cfm). The higher the number, the faster it cleans. That is how AHAM, the US association of home appliance makers that wrote the AC-1 test, defines it.
Why it matters: a purifier that is too small for the room is a common disappointment. In the UK forum threads we read, several owners found a small unit didn’t cope with a living room. The government’s SAGE-EMG advisers make the same point: a unit that works in a small room may do little in a large one.
What to look for: the CADR in m³/h, then our rule, room m² ≈ CADR ÷ 12. A room with a 2.4 m ceiling holds 2.4 m³ of air over each m² of floor, and five air changes an hour means 5 × 2.4 = 12 m³/h of clean air per m². So multiply your floor area by 12 for the CADR you need, or divide a CADR by 12 for the room it suits. Five changes is a little stricter than the 4.8 AHAM uses for its own room sizes (CADR ÷ 11.5), and above the minimum of three that AHAM says indoor-air researchers recommend. For a ceiling higher than 2.4 m, multiply again by your ceiling height ÷ 2.4: a 3 m ceiling needs a quarter more.
| Room | Size | Floor area | CADR needed | Our room-size band |
|---|---|---|---|---|
| Nursery or box room | 2.5 × 3 m | 7.5 m² | 90 m³/h | Small |
| Double bedroom | 3.5 × 4 m | 14 m² | 168 m³/h | Small |
| Small living room | 4 × 5 m | 20 m² | 240 m³/h | Small (top of the band) |
| Living room | 5 × 6 m | 30 m² | 360 m³/h | Medium |
| Through-lounge | 4 × 8.5 m | 34 m² | 408 m³/h | Medium |
| Open-plan kitchen and living room | 6 × 8 m | 48 m² | 576 m³/h | Large |
Small rooms, up to 20 square metres
Bedrooms, nurseries and studies need a CADR of up to 240 m³/h. The 14 m² double bedroom above needs about 170. Philips’ AC0650, at 170 m³/h, just covers it, with nothing to spare, and has no sensor or auto mode, so you choose the speed. The Coway Airmega 150, at 281 m³/h, suits about 23 m² and does the same job with room to spare. See every purifier in our bedrooms and small rooms band.
Medium rooms, 20 to 35 square metres
Most living rooms and large bedrooms fall here, needing 240 to 420 m³/h. The 5 × 6 m living room needs about 360. The Philips AC2220 states 420 m³/h, enough for up to 35 m², the top of the band, while the Winix T500, at 250 m³/h, covers only its bottom end, about 21 m². See our living-room purifiers.
Large and open-plan rooms, over 35 square metres
Anything above 35 m² needs more than 420 m³/h. The 48 m² open-plan room needs about 580: the Levoit Core 600S, at 697 m³/h, covers about 58 m², while the Winix T800 (500 m³/h, about 42 m²) and the Blueair Blue Max 3450i (481 m³/h for smoke, about 40 m²) fall a little short of it. If no single unit is big enough, two whose CADRs add up to what the room needs do the same sum. See our large and open-plan room purifiers.
Why brand room sizes look bigger than ours
What it is: a maker’s room size is its CADR divided by the number of air changes an hour it assumes and a ceiling height. The fewer air changes it assumes, the bigger the room on the box.
Why it matters: two purifiers with the same CADR can be sold for very different rooms. The “recommended room size” in our spec tables is the maker’s own figure, so read it with the basis below.
| Purifier | Maker’s CADR | Maker’s room size | Maker’s basis | CADR ÷ 12 |
|---|---|---|---|---|
| Philips AC2220 | 420 m³/h | 109 m² | NRCC-54013 method (about 1.6 changes an hour, our arithmetic) | 35 m² |
| Blueair Blue Max 3450i | 481 m³/h (smoke) | 54 m² and 43 m² | 4.8 changes an hour | 40 m² |
| Coway Airmega 150 | 281 m³/h | 29 m² in 15 minutes; 73 m² maximum | Four changes an hour; none given for 73 m² | 23 m² |
| Winix T800 | 500 m³/h | 120 m² | 4.8 changes, says Winix’s FAQ; the figure implies under two | 42 m² |
| Levoit Core 600S | 697 m³/h | 147 m² | Two changes an hour | 58 m² |
| IQAir HealthPro 250 | Not published | 104 m² | Two changes an hour, 2.4 m ceiling | About 42 m² at five changes |
Philips works out its room sizes with a method it cites as NRCC-54013, which comes to about 1.6 air changes an hour for the AC2220. Levoit and IQAir state two changes an hour outright. Coway’s “29 m² in 15 minutes” is four changes, but it gives no basis for its 73 m² maximum. Winix’s FAQ says 4.8 changes, yet 500 m³/h over 120 m² is fewer than two. Blueair sizes rooms on AHAM’s 4.8 changes, but its 3450i page gives two answers: on our arithmetic, 54 m² matches its pollen CADR and 43 m² is close to what its smoke CADR gives.
What to look for: the air-change rate in the small print. If there isn’t one, ignore the room size and divide the CADR by 12.
Which test is behind the CADR
What it is: you will meet three test methods on spec sheets.
- AHAM AC-1 (US). Particles are released in a 28.4 m³ chamber, and the rate their count falls is measured with the purifier off and then on; the CADR is the difference between the two rates, times the chamber volume. It gives three CADRs: tobacco smoke (about 0.09 to 1 micron), fine dust (0.5 to 3 microns) and pollen (0.5 to 11 microns). The chamber can measure up to 450 cfm, about 765 m³/h.
- IEC 63086-1 (international). The general method for measuring the performance of household air cleaners, published in the UK as BS EN IEC 63086-1:2020+A1:2023, with IEC 63086-2-1:2024 adding the part on reducing particles. AHAM says it was developed on the same chamber principles as AC-1.
- GB/T 18801 (China). The standard Philips quotes for its CADRs, in the 2022 edition for the AC2220. We haven’t read it, so we don’t claim its figures line up exactly with AHAM’s; we size Philips’ CADRs with the same rule as everyone else’s.
Why it matters: figures from different tests aren’t guaranteed to match, and most brands don’t say which test they used. In the UK material we read, only Philips names a standard for its CADRs. Blueair gives separate smoke, dust and pollen CADRs, the three AC-1 measures, but its UK page doesn’t name a test. Levoit, Coway and Winix name none for their UK CADRs. AHAM also points out what a CADR leaves out: it doesn’t show how well a purifier deals with bacteria or viruses, and there was no agreed test for gases or VOCs.
What to look for: a standard named next to the CADR. Where a maker gives three figures, size on the smoke CADR: it covers the smallest particles, and for the Blue Max 3450i it is the lowest of the three, so it is the cautious choice.
Converting cfm to cubic metres an hour
US listings, and some UK ones, quote CADR in cubic feet per minute. AHAM’s conversion is 1 cfm = 1.699 m³/h, so 200 cfm is about 340 m³/h. Levoit’s Core 300S manual gives both figures, 152 cfm and 258 m³/h, which agree. To go straight from cfm to a room size on our rule, divide the cfm by 7 for a close answer: 152 ÷ 7 is about 22 m², against 21.5 m² from 258 ÷ 12. AHAM’s own “2/3 rule”, which asks for a smoke CADR in cfm of at least two-thirds of the floor area in square feet, works out in metric at almost exactly our CADR ÷ 12.
CADR is a top-speed number
What it is: AC-1 tests a purifier on its highest speed only, so its CADR tells you nothing about how much it cleans on its quieter or sleep settings.
Why it matters: most people run a purifier on low, especially overnight, and low speeds move far less air. IQAir’s spec sheet shows the scale: the HealthPro 250 moves 50 m³/h of air on its lowest speed and 440 m³/h on its highest. A purifier sized exactly for your room will deliver much less than its CADR on a sleep setting.
What to look for: headroom, and the noise figures at both ends. Choose a purifier whose CADR ÷ 12 is comfortably above your floor area, then check how loud it is on its lowest and top settings. For scale, a comparison of six HEPA air cleaners cited by SAGE-EMG measured 27 to 35 dBA at minimum and 45 to 53 dBA at maximum. Philips quotes 13 dB for the AC2220 on its lowest setting and Coway 20 dB for the Airmega 150, though brands don’t all measure noise the same way, so treat small gaps with caution. For a bedroom, see our best air purifiers for a bedroom.
When there is no CADR at all
Some makers publish no CADR, so our rule can’t size them. We show “not published” rather than guess.
- Dyson publishes no CADR for any model we track. It uses its own whole-room test instead, run at maximum setting in an 81 m³ room, which is about 34 m² of floor under a 2.4 m ceiling on our arithmetic. Most of its range is in our purifier fans and heaters.
- Shark gives no CADR for the HP150UK, only a room size of 12 m² at 4.8 air changes an hour.
- IQAir gives airflow per speed and a room size at two changes an hour for the HealthPro 250, but no CADR.
- Meaco gives airflow, which is not a CADR, for the Arete models in our purifier dehumidifiers and humidifiers.
Airflow is the air a fan moves; CADR is the clean air that comes out, so it depends on the filter too. Without a CADR from a named test, a room size rests on the maker’s own method.
What each budget buys
Budget
Under £150
Mostly compact purifiers for bedrooms and small living rooms, with CADRs of about 120 to 280 m³/h. The better ones add a particle sensor, auto mode and an app; the simplest leave you to choose the speed. Check the CADR covers your room with some to spare.
Mid-range
£150–£300
Where most living-room and open-plan purifiers sit, from about 240 to 700 m³/h, with particle sensors and auto mode as standard. Most of the choice for rooms over about 20 m² is here.
Premium
Over £300
You mostly pay for extra jobs, such as heating, cooling or humidifying, or for a heavy-duty filter set, not for more clean air. Many models in this band publish no CADR, so our rule can’t size them.
Prices move, and an offer can shift a model into a lower band, so check the current prices on each product. Filters are a running cost on top: our guide to replacement filters lists the intervals and part numbers.
Our top picks
Three picks that cover a living room, a bedroom and an open-plan space, each with a stated CADR and a carbon layer for smells. Our full ranking is in the best air purifiers in the UK, and all 16 models are in our air purifier comparison.
-
Best overall
Philips PureProtect Quiet 2200 Series (AC2220/10)
Three-year filter and a 13 dB sleep setting at a mid-range price
- 420 m³/h CADR
- rooms up to 109 m²
- 13 dB(A) on sleep
-
Best for bedrooms
Coway Airmega 150 (AP-1019C)
Quiet Mark certified, 20 dB at its quietest, filters last a year
- 281 m³/h CADR
- rooms up to 29 m²
- 20 dB(A) on sleep
-
Best for large rooms
Levoit Core 600S
The highest CADR we list, 697 m³/h, with app and auto modes
- 697 m³/h CADR
- rooms up to 147 m²
- 26 dB(A) on sleep
How we choose
- Started from the room and the CADR, with one rule for every model: floor area × 12.
- Read each maker’s UK product page, spec sheet or manual for the CADR, the room size and the basis behind it, and noted which test standard, if any, it names.
- Checked prices at the brand stores and retailers we track, and show the date we checked.
- Revisit the picks when prices move or a model is replaced.
What we don’t do: lab-test every product. When we have used one ourselves, the review says so.
Room size questions
How do I work out what CADR I need?
Multiply the room’s length by its width for the floor area, then multiply that by 12. A 4 × 5 m room is 20 m², so look for at least 240 m³/h. That gives about five air changes an hour under a 2.4 m ceiling; if yours is higher, multiply again by your ceiling height divided by 2.4. For a bedroom, aim higher still, so the purifier can do the job on a quiet setting.
Is it worth buying a bigger air purifier than I need?
Usually, yes. CADR is measured on top speed, and few people run a purifier flat out for long. A unit with spare capacity can clean your room on a lower, quieter speed. The trade-offs are a bigger machine on the floor and more noise and power if you do turn it up: the Levoit Core 600S, for example, stands 60 cm tall and is rated at 49 W.
Can one air purifier clean the whole house?
Not well. A purifier cleans only the air that passes through it, so it works best in one room, sized for that room. Size it for the room you use most, often the bedroom, or add up the floor area of an open-plan space and size for the total.
Why doesn’t Dyson publish a CADR?
Dyson uses its own whole-room test instead. It bases its purifying claims on a test run at maximum setting in an 81 m³ room, rather than publishing a CADR from a standard such as AHAM AC-1. That makes its models hard to size against others, so our comparison shows their CADR as “not published”. Shark (HP150UK) and IQAir (HealthPro 250) publish none either.
Does CADR cover smells, gases or viruses?
No. AHAM says CADR measures particles: it doesn’t show how well a purifier deals with bacteria or viruses, and there was no agreed test for gases or VOCs. For smells you need activated carbon in the filter; the US EPA says purifiers without a sorbent such as carbon won’t remove gases. All three of our top picks above have a carbon layer.
