AC Filters: What Are MERV Numbers and What do They Mean?

Published on: July 20, 2012

In the world of AC repair and air conditioning maintenance, the acronym MERV stands for “minimum efficiency reporting value.” Many people believe that this value refers to the size of the filter holes in the air filter, but that is a common misconception.

In actuality, the MERV filter rating in an air conditioner is not a rating at all in that a higher number does not mean a better filter. This has to do with the misconception that many people have of a filter with smaller holes being less susceptible to AC repair or air conditioning maintenance and providing better indoor air quality. This assessment is simply not true and grossly simplifies the true meaning of the MERV number.

Filters do not actually work by catching all of the unwanted particles from the outside air. In actuality, less than 1% of particles that are stopped by an air filter occur because of the straining mechanism of that filter. The other 99% of particles are stopped because of techniques known as diffusion, impingement and interception. The best filters also use electrostatic attraction to stop unwanted particles from entering a living space.

In order to protect yourself from the contaminants of the outside air and from the undue expenditure of funds for AC repair and air conditioning maintenance, you must learn what the MERV number really means when it comes to air filters.


What actually separates a good filter from a bad one?

The performance of an air filter is actually due to the amount of particles that are counted moving down and upstream inside of the filter. This test must be performed a total of six times with ASHRAE Standard 52.1 test dust.

During this test, particles are created of a controlled size and placed into the air stream. The ASHRAE standard notes the size in microns of the particles that are created and assigns them all a range number. In total, there are 12 particle size range numbers.

For each particle range number, the procedure is performed. This will give an efficiency number for each of the 12 particle sizes. These efficiency numbers are averaged and placed into larger groups. It is from these percentages that the minimum efficiency reporting value is taken.

The minimum efficiency reporting value is also contingent upon the airflow speed that the filter is tested on. Although some filters do perform much differently at various airflow speeds, most of the tests that are performed on air filters are done at the same speed.

Because the particles that are used in a MERV test are created in a synthetic fashion, many people argue that the tests that are performed to actually get MERV numbers are inaccurate. They believe that the situations that are tested would never happen in the real world. There have been certain situations that have shown a drop in efficiency from the test to the real world. However, this drop in efficiency is usually not observable to the average consumer in the average living space. For the most part, the MERV ratings that are found correspond to the actual performance of the air filter in an average environment.

It is the job of the consumer and any professional help enlisted to get an air filter with a MERV rating that corresponds to the MERV rating that is needed for a particular living space. Matching these two variables will minimize the need for air conditioning maintenance calls. As a matter of fact, it has been found that most AC repair calls are needed as a result of the buyer disregarding or not knowing about MERV rankings.


Because of the nature of the tests described, it is not a good idea to necessarily correlate a higher MERV ranking with a better air filter. The way to use all MERV rankings correctly is to match the MERV ranking that an air filter has with the suggested MERV ranking that your living space needs.