Air Purifier CADR vs Room Size: How to Size It Properly

The easiest air-purifier number to misread is the advertised room size. A claim such as 100 m² or 130 m² looks decisive until you ask a more useful question: at what air-change rate?

For sizing, treat CADR, room volume and ACH as three connected inputs. The largest square-metre headline is rarely enough on its own.

CADR is the clean-air flow, not the room-size answer

CADR — clean-air delivery rate — is a flow rate for cleaned air. In PIQSY's governed Australian records it is normally expressed in cubic metres per hour, or m³/h.

A resolved CADR gives you a useful common input for sizing. It still needs the room volume and the turnover you want before it becomes a room-fit decision.

Ordinary airflow is not interchangeable with CADR. Dyson, for example, publishes 70 L/s airflow for HushJet Purifier Compact. That figure is not converted into CADR because total appliance airflow and clean-air delivery are different measurements.

Start with room volume, not floor area alone

Air cleaners work on a volume of air. A 30 m² room with a 2.4 m ceiling contains less air than the same floor area with a 3.2 m ceiling.

The basic geometry is:

room volume = floor area × ceiling height

A 30 m² room with a 2.4 m ceiling contains about 72 m³ of air. That simple calculation is more useful than assuming every 30 m² room is the same purification job.

ACH tells you how quickly that room volume turns over

ACH means air changes per hour. In a simplified sizing calculation:

approximate ACH = CADR ÷ room volume

Or, working backwards:

approximate floor area = CADR ÷ target ACH ÷ ceiling height

These are sizing calculations, not medical prescriptions. There is no single universal ACH target that is automatically right for every pollutant, room and person.

The same purifier can produce very different room-size claims

Assume a purifier with 480 m³/h CADR in a room with a 2.4 m ceiling. The same airflow works out to roughly:

• 200 m² at 1 ACH
• 83 m² at 2.4 ACH
• 50 m² at 4 ACH
• 40 m² at 5 ACH

Nothing about the purifier changed. Only the assumed turnover did.

That is why large manufacturer area claims should never be compared without their air-change basis.

Shark HP300 makes the trade-off unusually visible

Shark publishes 130 m² at 1 ACH and approximately 28 m² at 4.8 ACH for HP300ANZ.

Both claims can be true at the same time. The 130 m² headline describes much slower turnover; the 28 m² figure describes repeated air cleaning at a substantially higher rate. When repeated particle removal matters, that context changes how the product should be positioned.

There is no governed official HP300ANZ CADR, so one is not reverse-engineered from the room claims.

Dyson HushJet has the same 1-ACH caveat

Dyson's 100 m² HushJet Purifier Compact claim is explicitly based on 1 ACH. It should not be compared directly with another brand's room claim if that brand uses a different turnover assumption.

HushJet can still make sense for 24 dB Sleep Mode, 41 dB maximum purification noise, MyDyson controls and its conditional long filter-life proposition. Those features do not turn 100 m² at 1 ACH into evidence of high-turnover purification across 100 m².

A resolved CADR makes comparison cleaner, not automatic

Coway Airmega 400 has a resolved Australian 677 m³/h CADR. Levoit Core 400S has a resolved 433 m³/h CADR.

Those numbers are easier to use in room calculations than an area claim with no comparable ACH basis. They still do not make the higher-CADR model universally better: a smaller bedroom may not need Airmega 400-level throughput, while Core 400S adds connected controls another buyer may value more.

Size first, then compare the ownership features that still matter.

Conflicting CADR figures should stay unresolved

Current manufacturer material for Winix Zero+ PRO and Levoit Core 600S contains conflicting CADR figures. Averaging those values, choosing the largest one or quietly favouring the most flattering figure would create false precision.

When exact sizing depends on one resolved number, that evidence conflict is a legitimate reason to prefer a cleaner source record.

A practical sizing workflow

1. Measure the floor area and use a realistic ceiling height to estimate room volume.
2. Decide whether the job is gentle background cleaning or faster repeated turnover.
3. Prefer an exact-model resolved CADR when precise sizing matters.
4. Where only room size is published, keep the stated ACH basis attached to the claim.
5. Once airflow is adequate, compare noise, smart control, filtration architecture and maintenance.

For bushfire smoke, NSW Health advises that a HEPA purifier should be appropriately sized for the room and used in a small, well-sealed area to work effectively. The practical lesson is the same: room fit matters more than the biggest marketing number.

The shortcut to avoid

Do not reward the largest advertised m² claim, convert ordinary airflow into CADR, infer CADR from a room-size claim, or treat a 1-ACH coverage number as evidence of rapid particle removal.

Use PIQSY Match when you want room, pollutant priority, noise preference and control requirements considered together.