The Air Purifier Sizing Calculator tells you exactly what CADR (Clean Air Delivery Rate) your room needs — adjusted for your outdoor AQI level, health profile, pets and occupants. Most online calculators ignore outdoor AQI entirely. Ours accounts for it, because a home in an area with AQI 300 outside needs dramatically more purification power than one with AQI 30. Get your personalised recommendation below, then check your current outdoor AQI at AQI of My Location to keep the result accurate.
CADR (Clean Air Delivery Rate) measures the volume of filtered air an air purifier delivers per minute. The higher the CADR, the faster the purifier cleans room air. ACH (Air Changes per Hour) measures how many times per hour the purifier processes the entire room volume. The EPA and AHAM recommend a minimum of 4.8 ACH for effective purification. Our calculator starts at 4 ACH and increases the requirement based on your specific health factors and outdoor AQI level.
The outdoor AQI adjustment is the key differentiator of this tool. When outdoor AQI exceeds 150, more outdoor air infiltrates indoors through gaps in windows, doors and HVAC systems. Consequently, your purifier must work harder to offset this infiltration load. At AQI 200+, we recommend running your purifier continuously on maximum speed — not just during high-pollution hours.
Frequently Asked Questions
What CADR do I need for a 300 sq ft room?
For a standard 300 sq ft room with 8 ft ceilings (2,400 cubic feet), healthy occupants and moderate outdoor AQI below 100, you need approximately 160 CADR minimum. For a household with asthma or allergies, this rises to around 200 CADR. In high-pollution areas with outdoor AQI above 200, target 240+ CADR for the same room size.
What does CADR stand for and why does it matter?
CADR stands for Clean Air Delivery Rate. It is the single most important specification for comparing air purifiers — more reliable than room coverage claims, which manufacturers often inflate. CADR is measured independently by AHAM (Association of Home Appliance Manufacturers) in cubic feet per minute (cfm). Higher CADR means faster air cleaning. Always choose a purifier where the CADR exceeds your calculated minimum — the extra capacity means you can run at lower, quieter speeds and still achieve adequate ACH.
Is HEPA or ioniser better for air purification?
True HEPA filtration is significantly better for health than ionisers. HEPA H13 filters capture 99.97% of particles 0.3 microns and larger, including all PM2.5, dust mite allergens, mould spores and pet dander. Ionisers produce ozone as a byproduct — ozone is itself a lung irritant and an AQI pollutant. The EPA and American Lung Association explicitly recommend against ozone-generating air purifiers for indoor use. Choose True HEPA every time.
How often should I replace my air purifier filter?
Most HEPA filters need replacement every 6–12 months under normal use. In high-pollution areas with outdoor AQI frequently above 200 — such as Delhi, Ghaziabad or Patna during winter — filters load much faster and may need replacement every 3–4 months. Run your purifier at maximum speed during high AQI days to accelerate particle capture, and check filter condition monthly during peak pollution season.
Can one air purifier cover multiple rooms?
No — air purifiers are effective only in the room where they are placed, provided doors between rooms are closed. Air purifiers do not circulate clean air between rooms efficiently because house layouts disrupt airflow. For multi-room protection, use one purifier per occupied room. Prioritise the bedroom first — you spend approximately one third of your life there — then the living room and children’s rooms.