CADR Calculator — What Size Air Purifier Do I Need?
Updated: October 2026 · m² room sizes · m³/h CADR output · modern and Victorian ceiling heights
Enter your room size below. The calculator converts to required CADR in m³/h at your chosen air-change rate, then shows which purifiers from our tested models cover it.
What is CADR and why does it matter?
CADR (Clean Air Delivery Rate) measures how many cubic metres of air a purifier cleans per hour at a given fan speed. In the UK, m³/h is the correct unit — many listings use US CFM figures, which need converting (1 CFM = 1.699 m³/h).
The formula for the required CADR is straightforward: room volume (m³) × air changes per hour. A 13m² room with a 2.4m ceiling has a volume of 31.2m³. At 3 ACH, you need a purifier with at least 94 m³/h CADR — anything less and the air is not cycling fast enough to stay clean. See our full CADR explainer for the detail on measurement standards.
Which ACH setting should I use?
| Setting | Who it is for | Typical use |
|---|---|---|
| 2 ACH | General background use, low-pollution rooms | Living rooms with no specific trigger |
| 3 ACH | Recommended for most UK homes | Bedrooms, homes with pets or dust mite sensitivity |
| 5 ACH | Allergy sufferers, asthma, high pollen sensitivity | Allergy UK and ECARF guidance for sensitive households |
Worked examples — typical UK rooms
Required CADR in m³/h at 2, 3 and 5 ACH. Victorian ceiling height (2.7m) produces larger volumes than modern builds (2.4m) — a meaningful difference for older UK properties.
| Room type | Size (m²) | Ceiling | 2 ACH | 3 ACH | 5 ACH |
|---|---|---|---|---|---|
| Box room / single bedroom | 8m² | 2.4m | 38 m³/h | 58 m³/h | 96 m³/h |
| Double bedroom (modern) | 13m² | 2.4m | 62 m³/h | 94 m³/h | 156 m³/h |
| Double bedroom (Victorian, 2.7m) | 13m² | 2.7m | 70 m³/h | 105 m³/h | 176 m³/h |
| Living room (terrace) | 18m² | 2.4m | 86 m³/h | 130 m³/h | 216 m³/h |
| Open-plan kitchen/living | 30m² | 2.4m | 144 m³/h | 216 m³/h | 360 m³/h |
| Open-plan (Victorian conversion) | 30m² | 2.7m | 162 m³/h | 243 m³/h | 405 m³/h |
Why does this differ from the manufacturer's room-size claim?
Most manufacturers state a room-size coverage figure based on 1–2 ACH — the minimum needed to call the air "cleaned" rather than the rate needed to keep sensitive users comfortable. At 1 ACH, a 55m² room coverage claim becomes a 27m² room at 2 ACH and an 18m² room at 3 ACH.
This is not dishonesty — it is a marketing convention, and every major brand does it. The calculator above corrects for it. Our CADR explainer covers the ACH question in full, including why EN 1822 certification says nothing about room size.
Running costs for the model that fits your room
Once you know which CADR tier your room needs, compare the 3-year running costs in our running cost guide — electricity and filter costs for all models, at the current Ofgem rate.