What problem this solves
Air purifier marketing focuses on square-metre coverage and filter layers (HEPA, activated carbon). The actual metric that matters is CADR — clean air delivery rate, measured in cubic feet per minute (cfm) or cubic metres per hour (m³/h). CADR is the AHAM (Association of Home Appliance Manufacturers) standard. A purifier that’s marketed for 30 m² might have a CADR of only 150 m³/h, which is enough for an allergen room but not for a virus-controlled room. The calculator applies the right CADR for your pollutant and room.
The pollutant dimension is what most calculators skip. Allergens settle slowly, so ACH 3 (3 air changes per hour) is sufficient. Smoke and viruses stay suspended, so ACH 5–6 is needed to clear them within a reasonable time. Industrial contaminants may need ACH 8+. The calculator picks the right target per use.
Who it’s for
- Allergy and asthma sufferers choosing a bedroom purifier
- Pet owners dealing with dander
- Smokers wanting to clear residual smoke
- Workshop owners needing to clear VOCs from paints or solvents
Why ACH, not just area
The AHAM standard for air purifiers uses the “2/3 rule”: a purifier must deliver a CADR equal to at least 2/3 of the room’s volume in m³ per hour for effective allergen removal. That’s an ACH of about 0.67 — quite low. For more demanding pollutants, the ACH target rises. ASHRAE 62.2 recommends ACH 0.35 for general ventilation, but that’s for fresh-air intake, not air cleaning. For allergen removal specifically, ACH 3 is more realistic.
Limitations
- CADR is measured for specific pollutants (smoke, dust, pollen); virus CADR is often inferred.
- Filter loading affects CADR — a purifier at half filter life delivers ~80% of rated CADR.
- Room layout affects performance — purifiers work best in open layouts; walls and furniture reduce effective coverage.
- Noise output at the highest fan speed can be 50+ dB; bedrooms may need a quieter unit.
- For very large rooms (40+ m²), consider multiple units or a ducted system.
Practical rules of thumb
For a bedroom: target ACH 3–4. For a living room with pets: ACH 5. For a home office with strong outdoor pollution: ACH 5–6. Place them in the breathing zone (bedside height, sitting height) not on the floor. Run continuously or on auto-mode for best results. Replace filters per the manufacturer’s schedule; a clogged filter can drop CADR by 70%.