Getting the air purifier room size right matters more than any feature on the box. A unit that is too small runs flat out and still cannot keep up. The key number is the clean air delivery rate, or CADR, and a simple rule links it to your floor area.
Quick answer
- CADR measures how fast a purifier removes particles, usually at its highest fan speed.
- The US EPA lists minimum CADR for particle removal at about two thirds of floor area, for an 8 foot ceiling.
- For a 200 sq ft room, that is a CADR of at least 130 cfm, or roughly 220 m³/h.
- Higher ceilings or open plans need a bigger unit.
- Avoid purifiers that intentionally make ozone.
Air purifier room size: the CADR chart
The US EPA gives a sizing chart in its guide to air cleaners for the home. Also, it assumes an 8 foot ceiling. Indian makers often quote CADR in cubic metres per hour, so the table below shows both. One cfm equals about 1.7 m³/h.
| Room area (sq ft) | Minimum CADR (cfm) | Roughly in m³/h |
|---|---|---|
| 100 | 65 | 110 |
| 200 | 130 | 221 |
| 300 | 195 | 331 |
| 400 | 260 | 442 |
| 500 | 325 | 552 |
| 600 | 390 | 663 |
How to measure your room
Multiply the length by the width in feet. For example, a bedroom of 12 by 15 feet is 180 sq ft. If you measure in metres, multiply the area by 10.76 to get square feet. Then pick a purifier whose CADR is at or above the chart value for the next size up.
Some adjustments apply. Next, if the ceiling is higher than 8 feet, the EPA says to choose a unit sized for a larger area. For an open plan where rooms connect, consider the whole connected space. Then, if one unit cannot cover it, use more than one.
Check the filter, not just the number
A good CADR still needs a good filter. Digit’s buying guide recommends True HEPA, which it says captures 99.97 percent of particles as small as 0.3 microns. It advises avoiding labels such as HEPA-like or HEPA-type.
Filters also wear out. Also, Digit suggests replacing them every 6 to 12 months depending on local pollution, and warns that original replacements can cost between ₹1,500 and ₹6,000. So before you buy, check the price and availability of filters, because running costs add up.
Stay away from ozone
The EPA warns against air cleaners that intentionally produce ozone, which irritates the lungs. Devices with ionisers, electrostatic precipitators or plasma technology may emit ozone. The California Air Resources Board and the Association of Home Appliance Manufacturers publish lists of tested units. Next, check them when a brand makes such claims.
Noise and daily use
CADR is measured at the highest fan speed, which is also the loudest. Then, Digit says under 50 dB on high is acceptable, and ideally below 35 dB in sleep mode. If you will run it overnight, this matters as much as CADR.
Keep doors and windows closed while it runs, and place it with a few feet of space around it. Then use our guide to AQI categories in India to decide when to switch it to a higher speed. Winter heating adds its own load, so see how a room heater affects your electricity bill too.
A quick buying checklist
- Measure the room and note the ceiling height.
- Match the CADR to the chart, or go one size up.
- Choose True HEPA and confirm the filter price.
- Skip ozone-producing models.
- Check the sleep mode noise level.
The sizing chart and ozone advice come from the US EPA guide to air cleaners. Remember that a purifier helps with particles, not with gases, so it is one part of a healthy home rather than a cure.
FAQ
What CADR do I need for a 150 sq ft room?
The EPA chart lists 130 cfm for 200 sq ft and 65 cfm for 100 sq ft. So for 150 sq ft, aim for about 100 cfm or more, which is roughly 170 m³/h.
Does a purifier work in a room with open doors?
It works less well. Open doors join the space to the rest of the home, so the purifier must clean a bigger volume. Keep doors closed or use a larger unit.
How often should I change the filter?
Follow the maker’s schedule. Also, Digit suggests every 6 to 12 months depending on pollution, and filters in heavily polluted cities may need earlier changes.
