When it comes to air filters, it's important to understand that there are two sizes that you need to be aware of: the nominal size and the actual size. The nominal size is a rounded estimate used for consistency, while the actual size is the exact dimensions without rounding. It's worth noting that although you usually only have one, you may need more than one whole house filter for your home. Whole house filters usually go to an air handler, which is usually located next to the HVAC system itself.
In some homes, they also go directly to the ductwork. Whole house filters are always 4, 5 or 6 inches thick and the nominal size is only an estimate from the manufacturer. Usually, this number is shown on the side of the filter. The nominal size creates consistency across the board, adding peace of mind and less confusion for consumers.
This also makes it easy to compare different filter models. If an air filter is too big, you'll need to return it. DO NOT cut it to make it look good. Cutting a filter to fit it can create air gaps and contaminants can pass into the air. When buying your air filter online, it's important to know that the nominal size is a rounded estimate and not all filters will have the exact same size.
If the manufacturer measures the actual size, the variation in the numbers could confuse buyers. To check the size, turn off the system, remove the filter from the compartment, and look at the dimensions. For example, a filter labeled 10 X 12 X 1 is 10 inches long, 12 inches wide, and one inch thick. If the filter is too small, it will loosen and some of the air (and debris) will not pass through the air filter.
Most people won't be able to find the size of air filter they need at their local home improvement store. It's also important to note that die cuts (filter frames) are manufactured in bulk and purchased as raw material for making air filters. The exact size is always slightly smaller than the nominal size to allow the filter to fit properly into the groove or housing. To make sure you have the correct size filter, especially if you are replacing it with a different brand, it is important to confirm both sizes. If an air filter is too small, air gaps will be created and contaminants will bypass the filter material. Here's a fun fact: 70% of what Filterbuy sells is out of the 10 most common air filter sizes. When shopping for an air filter online or in-store, make sure you know both sizes: nominal and actual.
This will ensure that you get a properly fitting air filter that will keep your home clean and safe from contaminants.
