The calculator gives a sizing estimate. Always check the maker’s CADR data and your room layout.
A purifier that is too small struggles to clean your air. Use the calculator below to find the CADR your room needs.
Table of Contents
Your Room
Air changes per hour (ACH) is how many times the purifier fully cycles the room’s air in an hour. 4 ACH is the standard recommendation for allergy relief; hospitals use 6–8.
Minimum CADR you need
Floor area
144 sq ft
Room volume
1,152 cu ft
Air changes/hr
4
Real purifiers that clear this CADR
Required CADR = (Length × Width × Ceiling Height × ACH) ÷ 60 — the same method AHAM-certified purifiers are rated against. Room volume in cubic feet, divided into the hourly rate, tells you how much clean air the unit must deliver per minute.
How This CADR Calculator Works
CADR stands for clean air delivery rate. It tells you how many cubic feet of clean air a purifier delivers each minute. To size a purifier, work out the volume of your room, decide how many times an hour you want the air replaced (air changes per hour, or ACH), and divide by 60.
Formula: CADR = (length × width × ceiling height × ACH) ÷ 60
The calculator offers four ACH choices: 2 for a light refresh, 4 for allergy and asthma relief, 6 for heavy allergens, pets or smoke, and 8 for maximum, medical-style cleaning.
Here is where air cleaning is heading: stronger fans in quieter bodies, more independent testing and apps that track air quality in real time. More buyers now ask for the CADR number first instead of the room size on the box.
Worked Examples
- 12 × 12 ft bedroom, 8-ft ceiling, 4 ACH. Volume is 1,152 cubic feet. 1,152 × 4 ÷ 60 = 76.8, so aim for a CADR of about 77 or higher.
- 15 × 20 ft living room, 8-ft ceiling, 4 ACH. Volume is 2,400 cubic feet. 2,400 × 4 ÷ 60 = 160.
- 20 × 25 ft open space, 9-ft ceiling, 6 ACH. Volume is 4,500 cubic feet. 4,500 × 6 ÷ 60 = 450, which is more than any single unit in the picks list, so you would use two or more.
A Quick Rule of Thumb
Industry groups suggest a smoke CADR of at least two-thirds of the room’s floor area for rooms with 8-foot ceilings. A 300 sq ft room would need a smoke CADR of about 200. That works out to roughly 5 air changes an hour, which is a little above the 4 ACH most people use for allergies.
Read the Right CADR Number
Purifiers list CADR for smoke, pollen and dust. Use the smoke number for general sizing, since smoke particles are the smallest of the three. A pollen-only number overstates cleaning power for fine particles. For more on reading labels, see what is a CADR rating.
Things That Change the Answer
- Fan speed: CADR is measured at top speed. A quiet low setting delivers less, so size up if you plan to run it low.
- Filter age: a clogged filter moves less air.
- Open doors: the room is bigger than the calculator thinks.
- Placement: keep it away from walls and furniture so air can flow.
The downside is a label game. Box coverage claims often assume a full air change per hour or even less, so a purifier marketed for 500 sq ft may only meet the allergy standard in a much smaller room. Filters also clog, and CADR drops as they do. Expect more shoppers to pick a purifier that looks right on the box but falls short in a real room. Use the CADR math, then add headroom for pets, smoke or wildfire days.
Next Steps
Ready to shop? Start with the best air purifiers for home. Want more on room sizing? Read how to choose the right size air purifier and what size air purifier you need. The same room math sizes an air conditioner too, so try the BTU calculator.
FAQ
What CADR do you need for a 300 sq ft room?
About 160 at 4 air changes an hour with 8-foot ceilings. Aim for more if you have pets or smoke.
What does ACH mean?
Air changes per hour, the number of times a purifier replaces the room’s air in an hour.
Is a higher CADR always better?
It cleans faster, but it also costs more and may be louder. Pick one that meets your number at a speed you can live with.
Which CADR number should you use?
Use the smoke number, since smoke particles are the smallest.
Final Word
Do the math, add a little headroom and choose a purifier that meets your number at a speed you can live with.
What comes next: tighter labeling, more filter-life alerts and smaller, quieter units that still move a lot of air. Run the numbers below, then pick a purifier that meets them at a low fan speed.
Related Guides
Keep going with how to choose the right size air purifier, what is a CADR rating, what size air purifier you need, best air purifiers for home and BTU calculator.
