How is an air purifier’s recommended room size calculated? | Digital Trends


We love talking about the more advanced features of smart air purifiers, like the ability to automatically test your air quality and make adjustments, but there’s one basic spec that’s especially important when you’re buying one: Capacity.

Capacity is typically measured in square feet, but sometimes models only use the CADR (Clean Air Delivery Rate) airflow rating. Let’s take a look at how to calculate room size and compare it to the purifier capacity, so that you know you’re getting a purifier that’s large enough to fit your needs.

Calculating room size


If you are dealing solely in traditional square footage (see our sections below for more info), then your calculations are straightforward. Pull out a measuring tape, measure your room, and take the closest possible Length x Width.

For example, a 16-by-12-foot room would have a square footage of 192 square feet.

If your room is an odd shape, or you would just like to save some time finding a reliable calculator, This square footage calculator could help. You can specify the shape of a room to find more accurate measurements, and measure right down to the inch.

Using the CADR

Many air purifiers don’t use square feet as a way to show capacity, since this can be a bit relative and confusing. Instead, they turn to a more accurate scale, called the CADR. This number represents how much air the purifier can filter over a set period of time, assuming it has a clean filter. The larger and more powerful the purifier, the higher the CADR rating.

If you only have a CADR to work with, you can still match it to a room size with a little extra work. There are two different options:

AHAM (Association of Home Appliance Manufacturers): Multiply the CADR value x 1.5. This should give you the equivalent to the square footage covered by the air purifier. It’s best used with the smoke-based CADR, but not all purifiers will show what type of CADR they’re using, so you may need to work with what you’ve got.

GB-18801 Standard: An Asian standard that incorporates how well-sealed a room is. If your room has fair sealing, you would use CADR x 0.12. If your room is pretty drafty, you would use CADR x 0.08.

What about high ceilings?

Woman beside Winix Air Purifier.

Both square footage specs and the CADR are made assuming that the room has a ceiling that is 8 feet high. Of course, often this isn’t the case in larger rooms, which means that your calculations will be off, but that’s still air that your purifier will have to process.

There’s no easy way around this. You’ll have to treat the air in your room above 8 feet as a separate space. If it’s close to another 8 feet again, you can double your measurements for a quick fix. But if the ceiling is slanted or it’s less than 8 extra feet, that makes calculations more difficult. Do the best you can to estimate the ratio of the extra space to your original measurement: For example, if it looks like the ceiling is around 12 feet high for your 16 x 12-foot room, you may want to take your 192-square foot measurement and multiple it by 1.5 to get 288 square feet.

What is the ACH?

This is a rating that stands for Air Exchanges Per Hour, a broad standard used to judge the quality of air purifiers. You can find it with this process:

CADR per hour (CADR per minute x 60) / Cubic feet of your space (Length x Width x Height)

The ACH should be higher than 5 for effective air purifying.

What if I want an air purifier to cover multiple rooms?

Take the square feet of multiple rooms and add them together. This method can quickly outscale home air purifiers, which are often rated for one room or one larger space. You may want to consider a larger whole-house air purifier, or a couple of smaller purifiers for primary rooms. Remember, air won’t flow as easily between multiple rooms, which can make it difficult for one purifier to handle a larger house.

Are air purifiers a waste of money?

This question is subjective based on how allergic you are to air-based allergens, how worried you are about toxic particles in your environment, or how annoyed you are at lingering odors. Air purifiers can handle these issues.

Many units also come with humidifying capabilities if you live in a dry area. For the best value for your money, look for a purifier with HEPA filters that can handle very small particles, but HEPA filters do require regular replacements to be effective. We have guides to help you get the most out of your air purifier and how exactly air purifiers help with your allergies.

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