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MERV, or, Minimum Efficiency Reporting Value is a filter rating system devised by (ASHRAE) to standardize and simplify filter efficiency ratings for the public. The higher the MERV rating, the higher the efficiency of the air filters. Simply stated, a MERV 8 filter will remove smaller particles from the air than a MERV 7 filter.
Nominal vs. Actual Size
A filter is often identified by its "nominal" size. This is the actual size of the filter, rounded up to the nearest inch. This makes the filters easier to find and compare. However, it does differ from it's actual size, which are the exact measurements of the filter. Also, two filters with the same nominal dimension does not mean they have same actual dimension. These dimensions can vary by manufacturer or even by filter model. The industry tolerance for these variances are +/- 1/8" for both the width and length of the filter.