What is CADR?
The Clean Air Delivery Rate (CADR) measures how much filtered air a purifier delivers, in cubic feet per minute (CFM) or cubic meters per hour (m³/h). It is tested under the AHAM AC-1 standard for three particle types – tobacco smoke, dust and pollen. The smoke number is the most relevant for fine particles (PM2.5), viruses and wildfire smoke, so CleanCADR uses it as the basis for sizing.
Required CADR (CFM) = room volume (ft³) × target air changes per hour ÷ 60
Air changes per hour (ACH)
ACH says how many times per hour the full volume of room air passes through a filter. Around 4 – 5 ACH is a good target for offices, meeting rooms and classrooms; allergy sufferers benefit from 5 – 6; during wildfire smoke events or for infection control 6 – 12 ACH is recommended. The common AHAM "2/3 rule" – CADR at least two-thirds of the floor area in square feet for an 8 ft ceiling – corresponds to about 5 ACH.
Clean air per person
The ASHRAE 241 standard for control of infectious aerosols expresses targets per occupant. For offices it is about 30 CFM of equivalent clean air per person; for classrooms it is higher. In busy meeting rooms the per-person requirement often exceeds the ACH requirement, which is why the calculator shows both and uses the larger value.
Typical sizes
| Room | Area | CADR for 5 ACH (9 ft ceiling) |
|---|
| Private office | 120 sq ft | 90 CFM |
| Small meeting room | 200 sq ft | 150 CFM |
| Classroom | 750 sq ft | 560 CFM (2 – 3 units) |
| Open office zone | 1,500 sq ft | 1,125 CFM (4 – 5 units) |
Buying checklist
- Certified CADR – look for the AHAM Verifide seal; avoid products that only quote a "room size" without CADR.
- True HEPA (H13) filters capture 99.97 % of 0.3-micron particles; "HEPA-type" filters do not meet the standard.
- Noise – check the decibel rating at the speed you'll actually use. Under 45 dB is comfortable for offices.
- Running costs – replacement filters often cost more over five years than the purifier itself.
- Energy Star models use 25 – 40 % less electricity.
- Avoid ozone – skip ionizers and "ozone generators"; look for CARB certification.
Placement and maintenance
Place purifiers where air can circulate freely, at least 30 cm from walls, and near where people sit. Run them continuously during working hours, change filters on schedule, and keep doors closed when cleaning a single room. Portable purifiers complement – they don't replace – good ventilation with outdoor air.
Frequently asked questions
What CADR do I need for my room?
Multiply the room volume in cubic feet by the target air changes per hour and divide by 60. A 300 sq ft office with 9 ft ceilings needs about 225 CFM for 5 ACH.
How many air changes per hour are recommended for an office?
About 4 – 6 ACH of clean air for general offices and meeting rooms, and more during smoke events or for infection control.
Is it better to have one big or two small air purifiers?
Two smaller units placed in different parts of the room give more even air cleaning and can run more quietly.
How often should I change the filter?
Typically every 6 – 12 months for HEPA filters, more often in smoky or dusty environments. Pre-filters can usually be vacuumed monthly.
Do air purifiers remove viruses?
HEPA filters capture particles in the size range of respiratory aerosols. Purifiers reduce airborne concentration but are one layer alongside ventilation and other measures.
About CleanCADR
CleanCADR is an independent, free calculator for sizing portable HEPA air purifiers in offices, meeting rooms, classrooms and homes, based on AHAM and ASHRAE guidance. We are not affiliated with any air purifier manufacturer. It is not medical advice.
Every formula on this page runs entirely in your web browser. The numbers you type are never uploaded, stored or shared. Our goal is simple: give you an honest starting point so you can compare product specifications with confidence and avoid paying for capacity you do not need – or buying something too small to do the job.
The methodology combines widely published engineering rules of thumb, manufacturer data sheets and the electrical code practices that installers use every day. We review the assumptions regularly and update the calculator when typical product specifications change.
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Last updated: October 28, 2026
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