What CADR Rating Do I Need? The Complete UK Guide
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What CADR rating do I need is the most important question anyone buying an air purifier should ask — and the one most product listings make it hardest to answer. CADR (Clean Air Delivery Rate) measures how many cubic metres of clean air a purifier delivers per hour. It is the single most reliable indicator of whether a unit will actually work in your room, and once you understand how to use it, every air purifier buying decision becomes straightforward.
This guide explains what CADR means, how to calculate the right number for any room in your home, and maps every model we recommend to the rooms it will and won’t work in — with the maths shown clearly so you can verify it yourself.
| ✅ Key Takeaways CADR (Clean Air Delivery Rate) measures how many cubic metres of clean air a purifier delivers per hour. It is the single most important spec when choosing an air purifier — more important than brand, price, or marketing claims. The two-thirds rule: your purifier’s CADR (m³/h) should be at least two-thirds of your room’s floor area in m². For a 15m² bedroom, that’s a minimum CADR of 100 m³/h. For clinical allergen reduction — meaningful improvement for asthma, hay fever, or pet allergy sufferers — aim for 4–5 air changes per hour. Multiply your room volume (m³) by 4 to get the target CADR. Manufacturer room size claims are often optimistic. Always calculate from CADR directly rather than trusting the stated coverage area on the box. The Levoit Core 300S (CADR 141 m³/h) covers rooms up to 18m². The Levoit Core 400S (CADR 230 m³/h) covers up to 38m². The Levoit Core 600S (CADR 697 m³/h) covers up to 59m². |
What Is CADR and Why Does It Matter?
CADR stands for Clean Air Delivery Rate. It is measured in cubic metres per hour (m³/h) and tells you how quickly an air purifier can filter a volume of air. A unit with a CADR of 200 m³/h delivers 200 cubic metres of cleaned air every hour.
The reason CADR matters more than almost any other spec is that it combines two variables that independently tell you very little: filter efficiency and fan power. A purifier can have a perfect True HEPA filter but a weak fan that barely moves air through it — the CADR will be low and the real-world performance will be poor. Conversely, a powerful fan with a poor filter moves a lot of air but doesn’t clean it. CADR captures both in a single number.
CADR is typically measured separately for three particle types: dust, pollen, and smoke. Smoke particles are the smallest and hardest to capture, so the smoke CADR is usually the lowest of the three and the most conservative measure of performance. When comparing models, use the smoke CADR if only one figure is available — it gives the most honest picture of real-world filtration.
| ℹ️ CADR units: In the UK and Europe, CADR is typically expressed in m³/h (cubic metres per hour). Some US-spec product listings express it in CFM (cubic feet per minute). To convert CFM to m³/h, multiply by 1.699. For example, a CADR of 200 CFM = 340 m³/h. |
Two Rules for Calculating the CADR You Need
Rule 1: The Two-Thirds Rule (minimum effective threshold)
The most commonly cited CADR guideline is the two-thirds rule: your purifier’s CADR in m³/h should be at least two-thirds of your room’s floor area in m².
Formula: Minimum CADR = Room floor area (m²) × 0.67
Example: A 15m² bedroom requires a minimum CADR of 15 × 0.67 = 100 m³/h.
This rule assumes a standard ceiling height of approximately 2.4m and delivers roughly 2–3 air changes per hour (ACH) — sufficient for general air quality improvement but on the lower end for allergy or asthma sufferers.
Rule 2: The Four Air Changes Rule (recommended for allergy sufferers)
For meaningful allergen reduction — the threshold at which clinical studies show statistically significant improvement in symptoms for asthma, hay fever, and pet allergy sufferers — aim for 4–5 air changes per hour.
Formula: Recommended CADR = Room volume (m³) × 4
Room volume = floor area × ceiling height. For a 15m² room with 2.4m ceilings: volume = 36m³. Recommended CADR = 36 × 4 = 144 m³/h.
The Levoit Core 300S at 141 m³/h just meets this threshold for a 15m² bedroom — which is why it’s consistently our top recommendation for standard UK bedrooms.
| ℹ️ Which rule to use: Use the two-thirds rule as a baseline for general air quality improvement (dust, general pollution, odours). Use the four air changes rule if you are buying specifically for asthma, hay fever, pet allergies, or mould spore management — the clinical evidence for allergen reduction is based on 4+ ACH, not 2–3. |
CADR Requirements by Room Size
Using a standard 2.4m ceiling height, here are the minimum and recommended CADR values for common UK room sizes:
| Room | Floor Area | Min CADR (2/3 rule) | Recommended CADR (4 ACH) |
| Small bedroom | 8m² | 54 m³/h | 77 m³/h |
| Standard bedroom | 12m² | 80 m³/h | 115 m³/h |
| Large bedroom | 18m² | 120 m³/h | 173 m³/h |
| Small living room | 20m² | 133 m³/h | 192 m³/h |
| Medium living room | 30m² | 200 m³/h | 288 m³/h |
| Large living room | 40m² | 267 m³/h | 384 m³/h |
| Open plan kitchen-diner | 50m² | 333 m³/h | 480 m³/h |
Note: these figures assume the room is sealed (door and windows closed) during operation. An open door or window effectively increases the room volume and reduces the air changes per hour the unit delivers.
Why You Shouldn’t Trust Manufacturer Room Size Claims
Almost every air purifier box or product listing includes a stated room coverage area — ’60m²’ or ‘suitable for large rooms’. These figures are almost always calculated using 2 air changes per hour under ideal conditions, which is the minimum threshold for any measurable effect and well below the 4 ACH recommended for allergen reduction.
A unit claiming ’60m² coverage’ calculated at 2 ACH delivers the same air cleaning as a unit with half the coverage area calculated at 4 ACH. The room size claim tells you nothing useful on its own.
Always work from CADR directly. Find the CADR figure in the product specification sheet (not the marketing copy), divide by your room volume, and you’ll know exactly how many air changes per hour you’re getting. That number tells you whether the unit will work for your purpose.
| ⚠️ HEPA-type vs True HEPA: CADR figures on products labelled ‘HEPA-type’ or ‘HEPA-style’ are not directly comparable to CADR figures on certified True HEPA units. The filter efficiency is lower, meaning less of the air passing through the filter is actually cleaned. Always confirm the unit uses a certified True HEPA filter (99.97% at 0.3 microns) before relying on its CADR figure. |
CADR Figures for Every Model We Recommend
Here are the verified CADR figures for all air purifiers we recommend on this site, with real-world air changes per hour calculated for a typical 20m² living room (48m³ volume at 2.4m ceiling height):
| Model | CADR | Max Room Size | ACH in 20m² room |
| Levoit Core 300S | 141 m³/h | 18m² | 3.0× |
| Levoit Core 300 | 141 m³/h | 18m² | 3.0× |
| Levoit Core 400S | 230 m³/h | 38m² | 4.8× |
| Coway AP-1512HH | 246 m³/h | 41m² | 5.1× |
| Blueair Blue Pure 211+ | 540 m³/h | 50m² | 11.3× |
| Levoit Core 600S | 697 m³/h | 59m² | 14.5× |
The ACH column shows how many times per hour each unit clears the air in a 20m² room. The Levoit Core 400S and Coway AP-1512HH both exceed 4 ACH in a 20m² room — the clinical threshold for meaningful allergen reduction. The Core 300S falls just short at 3.0 ACH in a 20m² room, which is why we recommend it for rooms up to 18m² rather than larger spaces.
How Real-World Conditions Affect CADR Performance
Fan speed
CADR is almost always stated at maximum fan speed. At lower speeds, CADR drops significantly — sometimes by 50–70%. This matters because most people run their purifier on a low or medium setting for noise reasons, particularly overnight. The Levoit Core 300S delivers 141 m³/h at maximum speed but around 60–70 m³/h on its medium setting. Factor this in if quiet overnight running is important: size up one model to ensure you still hit your target ACH at the speed you’ll actually use.
Room sealing
Air changes per hour calculations assume a sealed room. Every gap under a door, open window, or through-wall ventilation point introduces unfiltered air and reduces effective ACH. In a well-sealed modern UK home this effect is modest. In an older, draughty property, it can be significant — particularly relevant for hay fever sufferers trying to keep pollen out during season.
Ceiling height
UK ceiling heights vary considerably — from around 2.2m in modern flats to 3m+ in Victorian properties. Higher ceilings mean more room volume for the same floor area, which reduces ACH. A 20m² Victorian living room with 3m ceilings has a volume of 60m³ rather than 48m³ — the Levoit Core 400S would deliver 3.8 ACH rather than 4.8 ACH. If you have high ceilings, recalculate using your actual ceiling height.
Furniture and obstructions
Large furniture reduces the effective air volume in a room and can impede airflow around the purifier. In a heavily furnished room, size up slightly from the theoretical CADR requirement. Equally, placing a purifier behind a sofa or in a corner significantly reduces its effective draw — always position with clear airflow on all sides.
Which Model Should I Buy? A Quick Decision Guide
| 🏆 For bedrooms up to 18m²: Levoit Core 300S The Levoit Core 300S at 141 m³/h delivers 4 ACH in a 15m² bedroom — the clinical allergen threshold. At 24dB on sleep mode it’s near-silent, and at 45W it costs around 3p per night to run. For the standard UK bedroom it’s the correct size without overspending on capacity you don’t need.No app needed? The Levoit Core 300 delivers identical CADR and filtration at a lower price — the only difference is no Wi-Fi. |
| 🏆 For living rooms 18–40m²: Levoit Core 400S or Coway AP-1512HH The Levoit Core 400S (230 m³/h) and Coway AP-1512HH (246 m³/h) both hit 4–5 ACH in rooms up to around 30m². The Coway adds an air quality sensor and auto mode; the Levoit has no ioniser and lower wattage (38W vs 77W). If you want set-and-forget monitoring, choose the Coway and disable its ioniser. If you want simplicity and lower running costs, choose the 400S. |
| 🏆 For large and open-plan spaces 40–60m²: Levoit Core 600S or Blueair Blue Pure 211+ The Levoit Core 600S (697 m³/h) and Blueair Blue Pure 211+ (540 m³/h) are both appropriate for large open-plan spaces. The Core 600S delivers more CADR at lower wattage (47W); the Blueair has a distinctive fabric pre-filter design and a loyal following for build quality. Either delivers 4+ ACH across spaces up to 50–60m². |
Frequently Asked Questions
What does CADR mean on an air purifier?
CADR stands for Clean Air Delivery Rate. It measures how many cubic metres of clean air a purifier delivers per hour (m³/h). A higher CADR means the unit cleans more air per hour, making it suitable for larger rooms or delivering more air changes per hour in a given space. It is the most useful single number for comparing air purifiers because it reflects both filter efficiency and fan power together rather than either in isolation.
Is a higher CADR always better?
A higher CADR is better up to the point where you have more than 5–6 air changes per hour for your room size — beyond that, you’re getting diminishing returns on air quality improvement while paying more for the unit and running costs. The sweet spot for most residential use is 4–5 ACH. That said, if you are between sizes, always go higher rather than lower — an oversized unit running on a lower fan speed is quieter, gentler on the filter, and gives you headroom for larger rooms or open doors.
How do I calculate CADR for my room?
Measure your room’s length and width in metres and multiply them together for the floor area (m²). Multiply by your ceiling height for the volume (m³). For the minimum effective CADR, multiply the floor area by 0.67. For the recommended CADR for allergy sufferers, multiply the room volume by 4. Example: a 4m × 5m room with 2.4m ceilings has a floor area of 20m² and a volume of 48m³. Minimum CADR = 13 m³/h. Recommended CADR for allergen reduction = 192 m³/h.
Why is the CADR on my purifier lower than the stated room size suggests?
Manufacturer room size claims are typically based on 2 air changes per hour under ideal conditions — a generous calculation that makes the coverage area look larger than it performs in practice. The CADR figure is the honest number. Calculate your own ACH from the CADR and your room volume to understand what you’re actually getting. A unit claiming 40m² coverage with a CADR of 150 m³/h delivers 3.1 ACH in a 40m² room (96m³) — below the 4 ACH threshold for meaningful allergen reduction.
Does CADR change at different fan speeds?
Yes — significantly. Manufacturers test and state CADR at maximum fan speed. At medium speed, CADR typically drops to 50–60% of the maximum figure. At low or sleep mode, it may be 30–40% of maximum. If you plan to run your purifier on a lower setting for noise reasons — particularly overnight — calculate your ACH based on the lower speed CADR rather than the maximum. A useful rule of thumb: if you’ll run it on medium most of the time, halve the stated CADR for your calculations and size accordingly.
What CADR do I need for hay fever?
For hay fever, target 4–5 air changes per hour in the room where you spend the most time — almost always the bedroom. Calculate your room volume (floor area × ceiling height) and multiply by 4 for your target CADR. For a standard 15m² UK bedroom with 2.4m ceilings (36m³), that’s a target CADR of 144 m³/h. The Levoit Core 300S at 141 m³/h meets this threshold. Run it continuously on medium-to-high speed throughout pollen season with the bedroom window closed for best results.
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