CADR measures the clean air an air purifier delivers per minute in CFM, and for most rooms you can size it by multiplying the floor area by about 0.67.
A room air cleaner can be rated brilliantly and still leave a smoky room smoky — because the number on the box describes the machine, not your space. The gap between those two things is exactly what CADR closes, and it takes about two minutes of arithmetic to use.
The metric stands for clean air delivery rate: the volume of clean air an air cleaner delivers every minute, usually written in cubic feet per minute.
What The CADR Number Actually Tells You
CADR combines two things a spec sheet usually separates: how much air the fan pushes, and how much of the pollutant the filter removes from it. The REHVA particulate-matter guidance describes it as airflow free of the pollutant that reaches the room, estimated as flow multiplied by removal efficiency.
That combination is why CADR beats raw airflow as a shopping number. A unit moving 400 CFM through a weak filter can deliver a lower clean-air rate than a tighter unit moving 250 CFM. The math rewards filtration as much as muscle.
The critical caveat is that CADR is pollutant-specific. Dust, pollen, and smoke each get their own figure, and they are not interchangeable — a cleaner that shines on pollen may be mediocre on smoke.
How To Calculate CADR For Your Room
The core formula is required CADR equals room volume times your target air changes per hour, divided by 60. Volume is length times width times ceiling height in feet; the result is the CFM to look for.
- Measure the room’s length, width, and ceiling height, then multiply the three for cubic feet.
- Pick a target air changes per hour. Guidance and calculators commonly use 3 to 6, with the higher end for allergies, smoke, or wildfire smoke outside.
- Multiply volume by that number, then divide by 60 to convert hours to minutes.
- Compare the result to the unit’s dust CADR and pick a cleaner that meets or beats it.
Worked example: a 12 ft by 20 ft room with an 8 ft ceiling is 1,920 cubic feet. At 4 air changes per hour, that is 1,920 times 4, divided by 60 — a required CADR of 128 CFM.
If you would rather skip the volume step, the common AHAM rule of thumb gets you close: multiply the room’s square footage by about 0.67. That same 240 sq ft room returns roughly 160 CFM, a slightly more generous figure than the 4-ACH calculation because the shorthand assumes an 8 ft ceiling. Once you have your target number, the tested air cleaner roundups at real-world air cleaner picks for small rooms let you match it against units that were actually run.
Where The Shortcuts Break Down
A sizing number is a target, not a promise. Furniture, closed doors, air leaks, and where you place the unit all move the real-world result, so a cleaner that technically clears the bar can still underperform in a cluttered, leaky room. Treat CADR as the entry ticket, not the outcome.
Two situations also outgrow the shortcuts. The 0.67 rule assumes a standard 8 ft ceiling; vaulted ceilings and open floor plans need the actual room volume, which is why the volume formula exists. And all of this guidance covers portable room air cleaners in single indoor rooms — CADR is not a whole-house HVAC metric and shouldn’t be used to size ductwork or a furnace filter.
One conversion worth keeping: 1 CFM is about 1.699 cubic metres per hour, useful if a unit’s specs arrive in metric.
| Sizing Method | Input Needed | Result |
|---|---|---|
| Volume formula | Length × width × ceiling height, plus target ACH | Required CADR in CFM |
| Area shortcut | Room square footage only | CADR ≈ 0.67 × area |
| ACH check | Required CADR and room volume | Achieved air changes per hour |
| Metric conversion | A CADR given in m³/h | Divide by 1.699 for CFM |
Frequently Asked Questions
Is a higher CADR always better?
No. A higher CADR cleans a given room faster and suits larger spaces, but an oversized unit runs louder and costs more to operate. Match the number to the room, then size upward slightly if you want a faster clean.
Which CADR should I use if the box lists three?
Does CADR tell me how clean the air will actually be?
Only in combination with room conditions. CADR tells you how much clean air the unit can deliver, but furniture, air leakage, and placement affect what you actually breathe. Sizing well and positioning the unit centrally gives you the closest match to the rating.
References & Sources
- Harvard Healthy Buildings. “Portable Air Cleaner Sizing Tool” Source for sizing by room and target clean-air level, and the dust-CADR recommendation.
- AHAM Verifide. “Portable Air Cleaners and Air Changes per Hour” Defines CADR and ties room sizing to CADR and ACH.
- REHVA. “Criteria for Room Air Cleaners for Particulate Matter” Describes CADR as airflow times removal efficiency for a specific pollutant.