59.5 m² with CADR 18 — 7.56 changes an hour
A 59.5 m² room with a CADR of 18 m³/min gets 7.56 air changes an hour — ample. Dust halves in 5.5 minutes.
- Room size
- 59.5 m² (18 pyeong)
- Room volume
- 142.8 ㎥
- Clean air rate (CADR)
- 18 ㎥/min
- Air changes per hour
- 7.56
- Time to halve
- 5.5 min
- Time to a tenth
- 18.3 min
- CADR needed for the target
- 11.9 ㎥/min
- Area this CADR covers
- 90 ㎡
- Smallest CADR that fits
- 12 ㎥/min
Dust falls off exponentially
Clean air removes what is left in proportion to how much is left, so the time to halve the dust does not depend on where it started — the same arithmetic as radioactive half-life. Multiply air changes per hour by the halving time and you always get 41.6 minutes. Getting down to a tenth takes 3.32 times as long.
Same room, other purifiers
Same purifier, other rooms
Where the advertised area parts ways with reality
The Korean standard (KS C 9314) measures the coverage area in a chamber with a 2.4 m ceiling, and its definition assumes natural ventilation is happening alongside. A closed room does not get that help, so taking the printed area at face value leaves you short. That is why the usual advice is to buy with headroom over your actual room. This chart leaves ventilation out and counts the purifier alone.
The two-thirds rule lands exactly on five
The common American shorthand is “CADR in cfm should be at least two-thirds of the room’s square footage”. Work it out at an 8-foot ceiling: (2/3) × A × 60 ÷ (8 × A) = 5, exactly. It is not a rule of thumb at all, it is five air changes an hour written differently. Eight feet is 2.44 m, essentially the same room as the Korean chamber’s 2.4 m, which is why both countries land on the same target.
How to read it
- Air changes per hour = clean air rate (m³/min) × 60 ÷ room volume (m³).
- Room volume is area × a 2.4 m ceiling — the height the standard tests at.
- The widely used target is five changes an hour.
- Time to halve the dust = 60 × ln2 ÷ air changes. At five an hour, that is 8.3 minutes.
- Getting to a tenth takes 3.32 times as long as halving.
- The advertised coverage area includes natural ventilation — a closed room has less headroom.
What this calculation leaves out
This assumes a closed room with no new dust being made. In practice it seeps in around windows, gets stirred up when people move, and is created afresh by cooking. A clogged filter drops the CADR too. And the maths assumes the air mixes evenly through the room, which it does not if the purifier sits in a corner.
Common questions
Q. Is CADR 18 enough for 59.5 m²?
It gives 7.56 air changes an hour, so: ample. That is above the target of five.
Q. How soon does the dust drop?
Halved in 5.5 minutes, down to a tenth in 18.3 — assuming a closed room with no new dust.
Q. How large a room does this CADR handle?
Up to 27.2 pyeong (90 m²) at the target rate.
Q. What should I buy for 59.5 m²?
On this chart’s scale, CADR 12 m³/min is the first that meets the target.