What Size Air Purifier Do I Need? [3 Steps]

Air purifiers come in all different sizes, so how can you know what size air purifier you need? In this article, we break down the three steps to know what size air purifier you need and how a metric called CADR can help with this.

Step 1: Measure Size (Volume) of Room or Space

First, we need to know how much air the air purifier needs to clean.

To do this, measure the area of the floor in square feet or square meters. Next, multiply this number by the height of the room or space to get the total volume. If the number calculated is in cubic feet, divide this number by 35 to get the volume of the space in cubic meters.

What size air purifier do I need?

Step 2: Calculate the CADR Needed to Clean the Room

Next, we need to calculate how much “power” is needed from an air purifier to clean the volume of air calculated in step 1. The amount of air an air purifier is able to clean is given by its CADR rating. The higher a CADR rating, the more air the air purifier can clean per hour.

To do this, we first need to multiply the volume of the space by 3. The number after multiplying the volume of the room (in meters cubed) by 3 is the CADR needed (cubic meters per hour) from a purifier to clean the area.

The reason we multiply by 3 is that most spaces should aim to have all air in the space cleaned 3 times every hour. Therefore, we need to find an air purifier that is powerful enough to clean this much air. For certain situations such as COVID-19 prevention, a number higher than 3 times an hour may be more optimal.

Step 3: Find Air Purifier(s) with a CADR at or Above Number Calculated in Step 2

CADR ratings are given in two measurements, either cubic meters per hour (CMH) or cubic feet per minute (CFM). The CADR needed for the space calculated above is in cubic meters per hour (CMH).

Note: If only the CFM is given, this can be converted to CMH by multiplying by 1.7.

The CADR of powerful air purifiers typically ranges from 500-900 m3/hour. If the CADR needed is over this number, multiple air purifiers may be needed.

Be aware, the ratings listed are for the purifier’s highest setting. Many air purifiers’ highest settings can be quite loud and you may not want to use them on the highest setting (see bonus below).

Simple Test: Is My Air Purifier Strong Enough?

If your goal is to reduce dangerous PM2.5 pollution and particles in your air, there is a simple test to make sure your air purifier is strong enough. Use a PM2.5 air quality monitor to test the air quality before and after turning the air purifier on (for an hour or so). If the air purifier is unable to reduce the PM2.5 concentrate to under 5 micrograms, consider buying additional air purifiers or one with a higher CADR.

Read More: The Best PM2.5 Air Quality Monitors

Blast Mini HEPA filter air purifier in office for protection against COVID coronavirus

Bonus Tip: Consider Noise Level

Many air purifiers can get loud on high settings. It is important to have an air purifier that is not a disturbance. You can find how loud an air purifier is by checking its noise level measured in dB. Anything over 60 dB will very likely be too loud for many situations.

Make sure to check the air purifier’s noise level on high, as some air purifier companies market noise levels on lower settings, which is misleading.

Real-World Example: What Size Air Purifier Do I Need?

Let’s go through the process of finding what size air purifier is needed and what CADR is needed. For this example, we will find the CADR needed for an air purifier to clean a 30 by 30 feet (9.1 by 9.1 meter) space with 9-foot (2.7 meter) ceilings.

Step 1 is to calculate the volume of the space in meters: 9.1 x 9.1 x 2.7=223 cubed meters.

Step 2 is to multiply the volume by 3: 223 cubed meters x 3=669. The CADR of the purifier needed for the space should be at least 669 m3/hr.

Step 3 is to find an air purifier (or combination of air purifiers) with a CADR close to 669 m3/hr.

Note, if the CADR rating uses CFM like the air purifiers below, multiply this number by 1.7 to get the CADR in m3/hr.

From the Smart Air product line, one Blast Mini air purifier would be suitable for this space.


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Smart Air is a social enterprise that creates simple, no-nonsense air purifiers and provides free education to protect people’s health from the effects of air pollution. We are proud to be the only certified B-Corp dedicated to fighting air pollution.