Air Purifier Size Calculator
Calculate the right air purifier CADR rating from room size and air changes per hour. Enter values for instant results with step-by-step formulas.
Reviewed by Daniel Agrici, Founder & Lead Developer
Formula
CADR (CFM) = (Room Volume in cubic feet x Air Changes per Hour) / 60
The Clean Air Delivery Rate needed equals the room volume multiplied by the desired air changes per hour, divided by 60 to convert from cubic feet per hour to cubic feet per minute. Room volume is calculated as length times width times ceiling height.
Worked Examples
Example 1: Bedroom Air Purifier Sizing
Problem:A bedroom is 12 feet by 10 feet with standard 8-foot ceilings. The occupant has mild allergies and wants 4 air changes per hour. What CADR is needed?
Solution:Room area = 12 x 10 = 120 sq ft\nRoom volume = 120 x 8 = 960 cubic feet\nCADR needed = (960 x 4) / 60 = 64 CFM\n\nAHAM rule of thumb: 2/3 x 120 = 80 CFM minimum\nRecommended CADR: at least 80 CFM to account for real-world conditions\nRoom category: Small Room
Result:CADR needed: 64 CFM (recommended: 80+ CFM for small bedroom)
Example 2: Living Room with High Ceilings
Problem:An open-plan living room is 20 feet by 18 feet with 10-foot ceilings. An asthma patient needs 5 air changes per hour. Calculate the required CADR.
Solution:Room area = 20 x 18 = 360 sq ft\nRoom volume = 360 x 10 = 3,600 cubic feet\nCADR needed = (3,600 x 5) / 60 = 300 CFM\n\nAHAM rule of thumb: 2/3 x 360 = 240 CFM (but this underestimates due to tall ceiling)\nThe 10-foot ceiling adds 25% more volume vs standard 8-foot\nRecommended: 300+ CFM commercial-grade purifier or two 150+ CFM units
Result:CADR needed: 300 CFM (large room with high ceiling requires commercial-grade unit)
Frequently Asked Questions
What is CADR and why does it matter when choosing an air purifier?
CADR stands for Clean Air Delivery Rate and is the single most important specification for comparing air purifiers. It measures the volume of filtered air delivered by the purifier in cubic feet per minute. A higher CADR means the unit can clean a larger room or clean a smaller room more quickly. CADR is tested by the Association of Home Appliance Manufacturers using standardized methods that measure particle removal for three categories: smoke (smallest particles, 0.09 to 1.0 microns), dust (1.0 to 5.0 microns), and pollen (5.0 to 11.0 microns). When selecting an air purifier, choose one with a CADR that matches your room volume and desired air changes per hour. An undersized unit will run continuously without adequately cleaning the air.
How many air changes per hour do I need for my room?
The recommended air changes per hour depends on your specific needs and health conditions. For general air quality maintenance in a healthy household, 2 air changes per hour is the minimum recommendation. For homes with allergy sufferers, 4 air changes per hour provides meaningful relief from pollen, dust mites, and pet dander. Asthma patients benefit from 5 air changes per hour to maintain consistently low particle counts. Rooms where someone is recovering from illness should target 6 air changes per hour to reduce airborne pathogen exposure. Hospital rooms typically maintain 6 to 12 air changes per hour depending on the ward type. Higher air changes require more powerful and typically more expensive purifiers, so balancing health needs with budget is important.
How do I calculate the right CADR for my room size?
The CADR calculation is straightforward: multiply your room volume in cubic feet by the desired air changes per hour, then divide by 60 to convert to cubic feet per minute. For a 15 by 12 foot room with 8-foot ceilings, the volume is 1,440 cubic feet. At 4 air changes per hour, you need a CADR of 1,440 times 4 divided by 60, which equals 96 CFM. A common rule of thumb from AHAM is that the CADR should be at least two-thirds of the room area in square feet, so a 180 square foot room needs a CADR of at least 120. This rule of thumb assumes standard 8-foot ceilings and moderate air changes. Rooms with higher ceilings, open floor plans, or higher pollution levels require upward adjustment.
Does ceiling height really affect air purifier sizing?
Yes, ceiling height directly and proportionally affects the air volume that needs to be cleaned. A room with 10-foot ceilings has 25 percent more air volume than the same floor area with 8-foot ceilings, requiring a proportionally higher CADR rating. Many air purifier recommendations only reference room square footage, ignoring ceiling height, which can lead to undersized units in rooms with cathedral ceilings, lofts, or tall commercial spaces. A 200 square foot room with standard 8-foot ceilings contains 1,600 cubic feet, while the same room with 12-foot ceilings contains 2,400 cubic feet, a 50 percent increase. For spaces with unusually high ceilings above 9 feet, always calculate based on cubic footage rather than relying on square footage guidelines.
References
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy