Air purifier CADR calculator: how much CADR does your room need?
This air purifier CADR calculator turns your room size into the clean air delivery rate you should look for on the box. CADR is the volume of purified air a machine delivers per hour in a standard test, and it is the one number that lets you compare purifiers fairly. Enter the room length, width and ceiling height, choose what you are cleaning for, and you get the CADR needed in both m³/h (the way Indian and Chinese brands list it) and cfm (the way the AHAM label does).
During North Indian winters, when the CPCB air quality index for Delhi-NCR often sits in the Very Poor (301–400) or Severe (401–500) bands, buyers ask whether a 300 m³/h machine is enough for a 12 × 14 ft bedroom. Type your room in, enter the purifier’s CADR, and the page tells you whether it meets the target, how many times an hour it turns over the air, and how many minutes it takes to clear most of the particles.
How to use the Air Purifier CADR Calculator
- Choose metres or feet and enter the room length, width and ceiling height.
- Pick a goal: the AHAM two-thirds rule for everyday dust and PM2.5, the stricter smoke rule for heavy smog, or your own air changes per hour.
- Add a safety margin if your filters are old or doors are often open.
- Enter the CADR of the purifier you are considering, in m³/h or cfm.
- Read the CADR you need, the air changes per hour, the verdict and the largest room your machine suits; copy or share the summary.
Air Purifier CADR Calculator features
Two units at once
Shows the target in m³/h and in cfm, so you can compare Indian listings with AHAM labels (1 cfm = 1.699 m³/h).
Three sizing rules
AHAM two-thirds rule, the smoke rule of CADR equal to room area, or a custom air-change target.
Ceiling height included
Taller rooms need more CADR, as AHAM advises; the figure scales with height above 8 ft (2.44 m).
Check a real product
Compares the purifier you like with the target and gives its air changes per hour and the biggest room it suits.
How the CADR is calculated
- Two-thirds rule. AHAM, the Association of Home Appliance Manufacturers, says the smoke CADR of an air cleaner should be at least two-thirds of the room’s floor area in square feet, assuming an 8 ft ceiling. A 120 ft² room needs a smoke CADR of at least 80 cfm.
- Smoke rule. For wildfire smoke AHAM recommends a smoke CADR equal to the room’s area in square feet. The same room needs 120 cfm. We use this stricter rule for heavy smog and stubble-burning haze.
- Ceiling factor. AHAM says higher ceilings need a unit rated for a larger room. We scale the need by ceiling height ÷ 8 ft when the ceiling is taller; we do not reduce it for lower ceilings. This scaling is our own simple extension.
- Air changes. Air changes per hour (ACH) = CADR in m³/h ÷ room volume in m³. The two-thirds rule works out to 5 ACH at an 8 ft ceiling. The custom mode multiplies your target ACH by the volume.
- Clean-up time. Time to remove 95% of airborne particles = ln 20 ÷ ACH hours, which assumes the air mixes perfectly and nothing new is being produced. Real rooms take longer.
CADR needed at a glance (8 ft ceiling, two-thirds rule)
Values come straight from AHAM’s rule of thumb (as read on ahamverifide.org on 30 October 2026). For a heavy smog week use the smoke rule, which needs 50% more.
| Room area | Smoke CADR needed | In m³/h |
|---|---|---|
| 100 ft² (9.3 m²) | 67 cfm | 113 |
| 150 ft² (13.9 m²) | 100 cfm | 170 |
| 200 ft² (18.6 m²) | 133 cfm | 227 |
| 300 ft² (27.9 m²) | 200 cfm | 340 |
Limits and good practice
- CADR is measured for smoke, dust and pollen in a lab chamber. It says nothing about gases, VOCs or odours; those need a different filter such as activated carbon.
- A purifier only works well with doors and windows mostly closed. Keep the filter fresh and run it on a high setting while AQI is high.
- Do not trust a brand’s “covers 500 sq ft” claim without the CADR; the test conditions behind such a claim are not always stated.
- The result is a planning guide. People with asthma or other conditions should talk to a doctor about their needs.
Air Purifier CADR Calculator FAQ
What CADR do I need for a 150 sq ft room?
By the AHAM two-thirds rule, at least 100 cfm of smoke CADR, which is about 170 m³/h, assuming an 8 ft ceiling. With a 10 ft ceiling this calculator scales it to about 212 m³/h.
Is 300 m³/h CADR enough for my bedroom?
For most bedrooms up to roughly 22 m² (240 ft²) with a 2.7 m ceiling, yes, by the two-thirds rule. For heavy smog use the smoke rule, which cuts the room size that 300 m³/h covers by a third (about 15 m² instead of 22 m² at a 2.7 m ceiling).
How do I convert CADR between cfm and m³/h?
Multiply cfm by 1.699 to get m³/h, or divide m³/h by 1.699 to get cfm.
Why is the room size on the box bigger than this calculator says?
Some makers quote a coverage figure that assumes a lower number of air changes than the AHAM rule. Compare CADR to CADR rather than the coverage claim.
What is a good number of air changes per hour?
About 5 is the AHAM benchmark (the two-thirds rule); 6 or more is common advice for wildfire or heavy smog days. These are rules of thumb, not health limits.
Does the calculator work for a whole house?
It is meant for one closed room. For an open-plan space, add the connected areas together, or use several units.