19-7/8x21-1/2x1 Air Filter by Glasfloss

There are three ratings systems for air filters that are the most common used.

1. MERV - "Minimum Efficiency Reporting Value" (Industry Standard Method)
2. FPR - "Filter Performance Rating"
3. MPR - "Microparticle Performance Rating"

MERV

MERV is an acronym for  “Minimum Efficiency Reporting Value". This is the industry standard by which air filters are rated and is the only rating system that is regulated at the national level. It is the measure of how efficient a filter is at removing particles of different sizes from the air that passes through it. The MERV scale was established by ASHRAE (American Society of Heating, Refrigeration and Air Conditioning Engineers) in 1987 to measure air filter effectiveness on a more granular scale than other rating systems.

The MERV scale ranges from 1-16, designating how effective an air filter is at removing particulates of varying sizes. The higher the MERV rating not only indicated how much of a specific size particle it can capture and at what efficiency, but also the size of the particles it can capture. For example, a MERV 8 filter can capture up to 20% of particulates in the 1.0-3.0 micron range and up to 70% of particulates in the 3.0-10.0 micron range. Alternatively, a MERV 11 filter can capture up to 65% in the 1.0-3.0 range, up to 85% in the 3.0-10.0 range but can also capture up to 20% in the tiny, 0.3-1.0 range where MERV 8 captures none.

FPR

FPR is and acronym for "Filter Performance Rating" and was designed by Home Depot for air filters that are sold in their stores including Honeywell. FPR is based on a color coded scale from 4-10 to designate the quality and effectiveness of an air filter to capture different particles. FPR is similar to MERV in how it is calculated but not nearly as specific or accurate. 

MPR

When you are looking to find a new filter, there are two different sizes that you will need to be aware of. The Nominal Size and the Actual Size. You will want to make sure you are purchasing the correct actual size as well as the nominal size. 

The Nominal Size is the simplified size that is used to label the filter, which makes searching for a filter much easier. You’ll want to note that not all filters with the same nominal size have the same actual size. Most filters measure slightly smaller than the size they are labeled as. For example, our 20x25x4 Glasfloss ZL Series MERV 10 filter (20x25x4 being the Nominal size) measures as 19½ x 24½ x 3¾ inches (which is the Actual size). There are a few exceptions to this rule, such as the 20x25x4 MERV 10 Glasfloss Replacement for Honeywell, which measures 20 x 24-7/8 x 4-3/8 inches as the actual size. Both filters are considered 20x25x4 filters, but they are not interchangeable. The nominal size may make it a lot easier to find your filter, but it can also make it easy to pick the wrong size if you aren’t careful. 

The Actual Size of the filter is the exact measurements if you were to get out a tape measure and measure it yourself. Some brands print this number on the filter in smaller print underneath the nominal size. It is important to be aware of the actual size of your filter when ordering for the first time, instead of only going by the nominal size.