The formulas behind the calculator
Nothing here is proprietary — you can reproduce every number with a pocket calculator.
Room volume (m³) = length × width × ceiling height
Required CADR (m³/h) = room volume (m³) × target air changes per hour
Clean air changes per hour = CADR (m³/h) ÷ room volume (m³)
Time to remove 90% ≈ 2.3 ÷ air changes per hour (hours) | 99% ≈ 4.6 ÷ ACH
1 CFM = 1.699 m³/h | 1 m³/h = 0.589 CFM | 1 ft = 0.3048 m
Assumptions and limits
- Well-mixed room. The decay times assume cleaned air spreads evenly. Obstructions, long room shapes and short-circuiting between outlet and intake make real performance slower — see how airflow affects indoor air quality.
- No ongoing source. The −90% and −99% times describe a one-off release, such as smoke after cooking. With a continuous source, concentrations settle at a plateau instead; doubling clean air roughly halves that plateau.
- Purifier only. Ventilation and natural settling add to the total removal rate, so this calculation is deliberately conservative.
- Connected spaces count. If a room is permanently open to another (open-plan kitchen and living room), add both volumes.
- Particles only. CADR describes particle removal. It says nothing about gases, odors or CO₂.
Which air change target should I choose?
| Target | When it fits |
|---|---|
| 3 ACH | Low-source rooms with good ventilation; background filtration where noise is critical |
| 4–5 ACH | General homes and offices. U.S. CDC ventilation guidance suggests aiming for 5 or more air changes per hour of clean air |
| 6 ACH | Bedrooms of allergy-sensitive people, rooms with pets, rooms next to busy roads |
| 8 ACH | Outdoor smoke episodes, dense meeting rooms, fast recovery after cooking |
| 10+ ACH | Specialised settings — design alongside ventilation, not instead of it |
A worked example
Bedroom, 4.0 m × 3.5 m, ceiling 2.6 m. Volume = 36.4 m³. At 6 ACH the requirement is 218 m³/h, or 251 m³/h with a 15% margin — about 148 CFM. A purifier rated 250 m³/h at a medium speed would suit; one rated 250 m³/h only at maximum speed probably will not, because it is unlikely to be quiet enough to run overnight. See noise explained.
What to do with the result
- Shortlist products whose CADR at a usable fan speed meets your figure.
- Check the test method behind the CADR — see CADR explained.
- Check the noise level at that speed, then the annual filter cost.
- Plan placement so intake and outlet stay clear — see the placement guide.
Working on a commercial project? Our office planning guide covers per-room targets, noise limits and maintenance, or contact us with your room list.