π¬οΈ Air Purifier Size Calculator
The CADR your room actually needs.
CADR and Air Changes Explained
Air purifiers are rated by CADR (Clean Air Delivery Rate), the cubic feet per minute of clean air they produce, measured separately for smoke, dust, and pollen. The number that matters is air changes per hour (ACH): how many times the purifier filters the room's full air volume each hour. The Association of Home Appliance Manufacturers recommends a smoke CADR at least two-thirds of the room's square footage for basic use, which works out to about 4 to 5 air changes per hour.
This calculator computes your room's cubic volume and converts your chosen ACH target into the required CADR: CADR = (length x width x height x ACH) / 60. For allergies, 4 to 5 ACH is the standard; asthma, pets, or heavy cooking push to 5; wildfire smoke demands 6 or more because the particle load is so high.
Matching a Purifier to Your Room
A 180-square-foot bedroom with 8-foot ceilings is 1,440 cubic feet. At 5 ACH you need a CADR of 120. Most compact purifiers deliver 100 to 150 CADR, so one unit handles a bedroom; living rooms of 400+ square feet need 300+ CADR units or two smaller ones. Two smaller purifiers often beat one big one for coverage, since clean air does not travel well around corners and furniture.
Beyond the Numbers
True HEPA (not HEPA-type) captures 99.97 percent of 0.3-micron particles. Activated carbon handles odors and VOCs that HEPA misses. Size the CADR first, then check noise: a purifier too small for the room must run on loud high speed constantly, while a properly sized one cleans quietly on low. Replace filters on schedule; a clogged filter can halve real-world CADR.
Placement and Filter Costs
A purifier only cleans the air it can reach, so put it where you spend the most time, usually the bedroom, with clear space around the intake. The common CADR rule of thumb is a smoke CADR at least two thirds of the room's square footage. Check filter replacement prices before buying the unit: cheap purifiers with $80 proprietary filters cost more over two years than pricier models with $30 filters. Vacuum the pre-filter monthly to extend HEPA life, and run the unit on high for an hour after cooking or cleaning, then drop to low for maintenance. For wildfire smoke or strong allergies, one correctly sized unit per lived-in room beats one oversized unit for the whole house.
Frequently Asked Questions
What CADR do I need for my bedroom?
A typical 12x15 bedroom with 8-foot ceilings needs about 120 CADR for 5 air changes per hour. Use the calculator with your exact dimensions and use case.
What is a good ACH for allergies?
Four to 5 air changes per hour is the standard recommendation. Asthma, pets, or cooking odors push to 5; wildfire smoke needs 6 or more.
Is HEPA-type as good as True HEPA?
No. True HEPA captures 99.97 percent of 0.3-micron particles and is the tested standard. HEPA-type is an unregulated marketing term with much lower performance.
Can one purifier cover my whole house?
Not effectively. Clean air does not travel well through doorways and around furniture. Size per room, or use two smaller units for large open spaces.
How often should I replace the filter?
Follow the manufacturer schedule, typically 6 to 12 months. In wildfire season or with pets, check sooner. A clogged filter can halve real-world CADR.