Air Purifier Sizing Calculator: The CADR Your Room Needs
Work out the smoke CADR your room needs from its size, ceiling height and target air changes, check AHAM's 2/3 rule, and see what your purifier delivers.
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This calculator tells you the smoke CADR an air purifier needs to clean your room at the air change rate you choose. For most rooms, choose five air changes an hour, which is close to the rate behind AHAM's own room sizes. For wildfire smoke, choose 7.5, which matches AHAM's advice to buy a smoke CADR equal to the room's floor area. Enter your purifier's AHAM smoke CADR to see how many air changes it gives your room and the largest room it can cover.
How much CADR does your room need?
8 ft is the standard the AHAM rule assumes.
From the AHAM Verifide seal or directory.
Smoke CADR needed
120 cfm
for 5 air changes an hour
AHAM 2/3 rule
120 cfm
180 sq ft floor
Your purifier gives
6.0 ACH
in 1,440 cu ft
Largest room it covers
215 sq ft
at 5 ACH, 8 ft ceiling
Room volume = length x width x ceiling height. CADR needed (cubic feet per minute) = volume x air changes per hour / 60. The AHAM rule of thumb is a smoke CADR of at least two thirds of the floor area for an 8 ft ceiling, scaled here for taller ceilings. Air changes your purifier gives = its smoke CADR x 60 / room volume. Use the smoke CADR from the AHAM Verifide directory or the box; it is the hardest of the three ratings and the one that matters for fine particles.
How the calculator works
A clean air delivery rate (CADR) is the volume of clean air a purifier delivers, in cubic feet per minute. Air changes per hour (ACH) is how many times that clean air replaces the room's volume every hour. The two are linked by one formula:
- Room volume (cubic feet) = length x width x ceiling height.
- CADR needed (cfm) = room volume x ACH / 60.
- ACH your purifier gives = its CADR x 60 / room volume.
- Largest room at a given ACH = CADR x 60 / (ACH x ceiling height), in square feet.
Where the defaults come from
| Setting | Default | Source |
|---|---|---|
| Room | 15 x 12 ft, 8 ft ceiling | A typical main bedroom; 8 ft is the ceiling AHAM's sizing assumes |
| Air changes per hour | 5 | AHAM's suggested room sizes are based on 4.8 ACH under its AC-1 standard; the CDC's NIOSH guidance suggests 5 or more ACH of clean air in occupied buildings |
| Wildfire option | 7.5 | AHAM recommends a smoke CADR equal to the room's floor area for wildfire smoke, which is 7.5 ACH at 8 ft |
| Your purifier's CADR | 143 cfm | The Levoit Core 300's AHAM smoke CADR, a common small-room purifier |
AHAM's two-thirds rule
AHAM's rule of thumb, on its guide to choosing an air cleaner, is that the purifier's smoke CADR should be at least two thirds of the room's floor area in square feet: a 10 by 12 ft room (120 sq ft) needs a smoke CADR of at least 80. The rule assumes 8 ft ceilings, and AHAM notes that higher ceilings need a purifier rated for a larger room. The calculator scales the rule for your ceiling height.
For wildfire smoke, AHAM recommends going further, to a smoke CADR equal to the floor area. Our wildfire smoke picks are sized on that basis.
Worked examples
| Room | Size and ceiling | Volume | 5 ACH | 7.5 ACH |
|---|---|---|---|---|
| Nursery | 10 x 11 ft, 8 ft | 880 cu ft | 73 cfm | 110 cfm |
| Bedroom | 12 x 15 ft, 8 ft | 1,440 cu ft | 120 cfm | 180 cfm |
| Living room | 15 x 20 ft, 9 ft | 2,700 cu ft | 225 cfm | 338 cfm |
| Open plan | 20 x 30 ft, 10 ft | 6,000 cu ft | 500 cfm | 750 cfm |
Which CADR to enter
Use the smoke CADR from the AHAM Verifide directory or the AHAM seal on the box. AHAM certifies three CADRs, smoke, dust and pollen, and smoke is the lowest for most purifiers because smoke particles are the hardest to catch. If you are sizing mainly for pollen or pet dander, the pollen CADR is a fair guide, but sizing on smoke CADR covers you on every count. Some makers, including Coway, Dyson, Alen and IQAir, were not in AHAM's directory when we checked; their stated CADRs work in the calculator but are the makers' own figures. Our CADR guide explains the three ratings.
When the calculator's answer is not enough
- Open doors and open plans. If the room opens onto another space, include both in the size.
- Sources inside the room. A smoker, a wood stove or frequent frying adds particles faster than an average room; size up.
- Noise. CADR is measured on the top speed. Buying about 1.5 times the CADR you need lets you reach your target on a quieter setting.
- Filter loading. A clogged filter delivers less than its rated CADR. Change filters on time.
Once you know the CADR you need, see our best air purifiers or the large-room picks.
Frequently asked questions
How do I calculate what size air purifier I need?
Multiply length, width and ceiling height to get the room's volume in cubic feet, multiply by the air changes per hour you want, and divide by 60. For a 12 by 15 ft room with 8 ft ceilings at 5 ACH, that is 120 cfm of smoke CADR.
What is the 2/3 rule for air purifiers?
AHAM's rule of thumb: the smoke CADR should be at least two thirds of the room's floor area in square feet, for 8 ft ceilings. A 120 sq ft room needs at least 80 cfm.
How many air changes per hour should an air purifier give?
About five for everyday use; AHAM's suggested room sizes are based on 4.8. For wildfire smoke, AHAM recommends a smoke CADR equal to the floor area, which is 7.5 air changes an hour at 8 ft.
Does ceiling height affect air purifier size?
Yes. A purifier cleans volume, so a 10 ft ceiling needs 25% more CADR than an 8 ft ceiling for the same floor area and air changes.
Can an air purifier be too big for a room?
Not for air quality; it just runs on a lower, quieter setting. The only downsides are price, size and filter cost.
Which CADR should I use: smoke, dust or pollen?
Smoke, the lowest and most demanding of AHAM's three ratings for most purifiers, so a purifier sized on smoke CADR is sized for all three.
Sources
- How to choose the best air cleaner for your home, AHAM Verifide
- Using an Air Cleaner to Manage Wildfire Smoke, AHAM Verifide
- AHAM Verifide Directory: Certified Room Air Cleaners, Association of Home Appliance Manufacturers
- How Much Ventilation Is Enough? Aim for 5, CDC, National Institute for Occupational Safety and Health
- Guide to Air Cleaners in the Home, US Environmental Protection Agency
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