How to Calculate CADR From an Air Purifier’s Airflow

CADR tests, used to calculate an air purifier’s effectiveness at cleaning air, are typically used to compare the performance of different purifiers. However, performing a CADR test is not simple: it requires a particle monitor, a sealed 30m3 test lab, and tightly controlled testing conditions.

As such, a shortcut to calculate an air purifier’s CADR – without the need to pollute a room or purchase expensive equipment – would be extremely helpful. In recent months, one such approach has been gaining some traction: multiplying an air purifier’s airflow by its filtration efficiency to calculate CADR. Here’s one tweet explaining how to calculate it:

On the surface, this approach seems far simpler and easier to do than running a CADR test, however several factors mean this method cannot be used to directly compare with CADR values calculcated using one of the CADR standards.

Can Airflow Values be Converted to CADR Values?

In short: no. Long answer: you can calculate an estimated CADR for a given machine using this method, useful for internal comparisons, but it should not be used to compare with existing purifiers tested using one of the CADR standards.

4 Things to Watch Out For When Using Airflow to Estimate CADR

  1. Air mixing in a real room should be taken into account. In an ideal world, dirty air would pass through an air purifier, get cleaned, and then wouldn’t need to pass through the purifier again. However, due to the way air mixes in a room this doesn’t happen. The clean air from the purifier mixes with existing dirty air in the room (like stirring sugar into our morning coffee), and will then pass once more through the purifier. That means some of the already-clean air to pass through the purifier a second or even third time. Re-purifying already purified air is not optimal, but it is inevitable. And that leads to a CADR that’s lower than just airflow x efficiency.
  2. A filter’s filtration efficiency varies with air speed. The manufacturer listed efficiency of a filter is typically tested at low air speeds 0.53cm/s. At higher air speeds, filtration efficiency typically drops off. Purifiers typically have airspeeds of 2-3m/s through the HEPA filter. In these cases, the filter’s filtration efficiecny may have dropped dramatically.
  3. Simply multiplying airflow by filtration efficiency does not account for different particle sizes. For example, a MERV13 will capture 90% of particles over 3 microns, but only ~50% 0.3-1.0 microns in size. Depending on which particle range you use for your filtration efficiency, the calculated CADR using this method can vary drastically.

How Much Lower Is CADR Than Airflow?

At Smart Air, we use airflow and CADR tests when testing purifiers like the Blast air purifiers. We use both because each has its own advantages. Airflow tests give a quick & rough estimate of a purifier’s effectiveness; CADR is more precise. Because we do both types of tests, we can see how CADR and airflow values are on average for a range of purifiers.

CADR vs airflow of air purifier

From this range of 5 air purifiers, we saw that tested CADR values were just 60% of tested airflow results. The drop was greater for the Blast and Blast Mini. This 60% value can be used as a handy rule of thumb where CADR values aren’t available. However, it may not be applicable to all air purifiers. If in doubt, it’s recommended to obtain CADR values directly via testing.

Read More: Steps to Test an Air Purifier’s CADR

DIY Air Purifier Tests

There have been a lot of great tests run by researchers in Europe and the US comparing CADR numbers with airflow x efficiency numbers for DIY air purifiers. The tweet above is just one example. A group of Spanish researchers suggested using a correction factor of about 10% – slightly less than the 40% suggested by Smart Air’s data above.

Read More: How we test out air purifiers at Smart Air

Bottom Line: How to Calculate CADR From an Air Purifier's Airflow

“Airflow x efficiency” can be used as a way to quickly estimate a purifier’s CADR. However, it should not be used to compare purifiers, unless they were all tested using the same method. When using “airflow x efficiency” as a quick rule-of-thumb, watch out for the 4 factors that may affect this, and you may need to use a correction factor.

Smart Air


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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.