To determine the right size air purifier for your room, you need to measure the square footage of the room you want to purify. The easiest way to do this is to use a formula that takes into account the Clean Air Delivery Rate (CADR) of the air purifier and the square footage of the room. Here are some steps to follow:

1. Measure the size (volume) of the room or space in square feet or square meters. Multiply this number by the height of the room or space to get the total volume in cubic feet or cubic meters.
2. Determine the CADR needed to clean the room. Ideally, you should select an air purifier that offers between 2 to 5 air changes per hour. With this rating, you can be assured your air purifier is cleaning the air every 12 to 30 minutes. For homes with pets or smokers, go for the maximum of 5 air changes per hour to significantly reduce the number of allergens. The CADR of the purifier needed for the space should be at least two-thirds of the room size. You can use a formula to calculate the CADR needed based on the square footage of the room.
3. Find an air purifier (or combination of air purifiers) with a CADR at or above the number calculated in step 2.

Air purifiers come in different sizes, and each size is good for a specific room size measured in total square feet. Here are some guidelines to follow:

• Small air purifier: Good for personal spaces and rooms up to 200 square feet.
• Medium air purifier: Good for rooms between 200 and 400 square feet.
• Large air purifier: Good for rooms between 400 and 1,500 square feet.
• Whole-house air purifier: Good for purifying every room in the house.

Its important to note that some air purifiers only show CFM (cubic feet per minute) of airflow, which will be higher than the CADR. To determine the required CFM, take the square footage of the room, divided by 30.

In summary, to determine the right size air purifier for your room, you need to measure the square footage of the room and use a formula that takes into account the CADR of the air purifier and the square footage of the room. You can then choose an air purifier that meets or slightly exceeds the square footage of the room you want to purify.