CADR is the Clean Air Delivery Rate. Choose a purifier with a smoke CADR at or above the result.
How to use this calculator
Enter the room size and ceiling height and choose how many times per hour you want the air to be cleaned. Press Calculate to see the CADR a purifier needs to clean the room.
The formula
Room volume = length × width × height
CADR (cfm) = volume × air changes per hour ÷ 60
2/3 rule: CADR ≥ two thirds of the floor area (for 8 ft ceilings)
Example: A 15 ft × 12 ft room with 8 ft ceilings is 1,440 cubic feet. For 4 air changes an hour you need a CADR of 96 cfm. The 2/3 rule gives 120 cfm.
What your air purifier size result means
The calculator gives a target CADR in cubic feet per minute (cfm). CADR is the clean air delivery rate, the volume of filtered air a purifier puts out. If a unit’s smoke CADR meets or beats your number, it can clean the whole room the chosen number of times each hour. The 2/3 rule result is a second check based on floor area. When the two numbers differ, buying for the higher one is the safer choice.
What changes the size you need
- Room volume. Length, width and ceiling height all count. A 15 by 12 foot room with 8 foot ceilings holds 1,440 cubic feet. Raise the ceiling to 10 feet and it holds 1,800 cubic feet, so the same purifier has more air to clean.
- Air changes per hour. Four is fine for general use. Allergies, pets or smoke call for five or six. Moving that 1,440 cubic foot room from 4 to 6 changes raises the target from 96 cfm to 144 cfm.
- Open floor plans. If a room opens onto a hallway or kitchen with no door, air mixes with the space next to it. Measure the connected area or size up.
Practical tips and common mistakes
- Compare the smoke CADR, not the pollen or dust CADR. Smoke particles are the hardest to catch, so the smoke number is usually the lowest and the most realistic.
- Be careful with room size claims on the box. Some are based on fewer air changes than you might want.
- CADR is measured on the highest fan speed. Most people run purifiers on a quieter setting, so a unit rated well above your target can run slower and still keep up.
- Keep the purifier a foot or two from walls and furniture so air can flow in and out.
- Close windows and doors while it runs, or it will be cleaning outdoor air too.
Cost and planning
The purchase price is only part of the cost. Replacement HEPA filters usually need changing every 6 to 12 months, and some models also use a carbon pre-filter that wears out sooner. Check the filter price and how easy it is to find before you buy. Energy use is typically low, but a unit that runs around the clock adds up over a year, so look at the wattage on each speed. For a large room, two medium purifiers placed at opposite ends can sometimes clean more evenly than one large unit.
