Air Purifier sizing
Air Purifier Sizing Guide & Calculator
A purifier rated CADR 200 is the right size for a 300 sq ft bedroom. For a 500 sq ft living room with pets, you want CADR 500 — twice the airflow. The AHAM 2/3 rule (CADR ≥ 2/3 of sq ft) is the minimum; allergies need a 1:1 ratio. Our calculator applies both.
Did you know? An air purifier rated 400 CADR in a 250 sq ft room gives you roughly 6 air changes per hour — the EPA-recommended level for asthma and allergy sufferers. Lower ACH and you’re filtering yesterday’s air.
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Air Purifier sizing FAQ
How do I choose the right air purifier size?
Match the CADR rating to your room square footage using the AHAM 2/3 rule: your purifier's CADR should be at least two-thirds of your room's sq ft. A 300 sq ft bedroom needs CADR 200; a 500 sq ft living room needs CADR 330. Use our free air purifier calculator for an exact recommendation.
What CADR rating is good for allergies?
For allergies and asthma, target a CADR that delivers 4.8+ ACH (air changes per hour) for your room. This typically means a CADR equal to your room's full square footage (not just 2/3). Look for purifiers with separate dust, pollen, and smoke CADR scores — the pollen CADR matters most for seasonal allergies.
Do I need a HEPA filter?
True HEPA filters capture 99.97% of particles down to 0.3 microns — this covers pollen, dust mites, pet dander, and most bacteria. For allergies or asthma, True HEPA is essentially required. For general air quality, less aggressive filters can work but HEPA is the gold standard recommended by the EPA.
Where should I place an air purifier?
Place it 6-12 inches from walls in the room's main living area, ideally where airflow is unobstructed. For bedrooms, position it across from the bed for cross-room airflow. Avoid corners (restricts airflow) and direct sunlight (some sensors are light-sensitive). For whole-home coverage, use multiple units rather than one oversized unit.
How often do I need to change the filter?
Most HEPA filters last 6-12 months in normal use; activated carbon pre-filters last 3 months. Pet owners and heavy smokers may need to replace filters more often. Many purifiers have filter-life indicators — use them. Running a purifier 24/7 shortens filter life vs. intermittent use.
Is ozone-producing air purifier safe?
No. Avoid ozone generators entirely — ozone is a lung irritant and is not effective at removing most indoor pollutants. The California Air Resources Board (CARB) certifies ozone-safe purifiers; AHAM-certified purifiers never produce harmful ozone. Ionizers are also controversial for asthma sufferers.
Standards used: AHAM AC-1 CADR Standard • EPA Indoor Air Quality • ENERGY STAR • Editorial policy • Methodology
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