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Properly Fitting HVAC Filter Replacements - Actual Vs. Nominal Size

If you've ever looked for filters online, you've probably come across the terms "actual size" and "nominal size". Even without knowing much about filters, you were probably able to deduce that these terms referred to the exact versus rounded dimensions of a filter. But when it comes to purchasing a filter, which size should you pay attention to? And why do filter manufacturers use both actual and nominal when describing the size of their products?

In order for an air filter to fit into its provided slot, it needs to be just slightly smaller than the size of the allotted space. This is where "actual" and "nominal" come in. Manufacturers round up and down in order to provide consistency. The resulting nominal dimensions are all you'll need to help you get started when replacing an HVAC filter.

The easiest way to decide which nominal filter size is right for your system is simply to remove its existing filter. On its side, you'll find three printed numbers: the nominal length, nominal width and nominal thickness. If you see two sets of numbers, the manufacturer has included the actual size of the filter as well.

Nominal measurements are actual measurements rounded up or down to the nearest inch. That means your filter is going to be either slightly smaller or slightly larger than what's printed on it. For that reason, it's also important to make a note of how well your existing filter fits your unit's space. If the filter is bent, it's too big. If there is more than 1/2" of space around your filter, it's too small. If your existing filter fits, simply stick to the nominal dimensions printed on its side when choosing your next filter replacement.

If your existing unit does not have a filter, you can measure the inside space of the filter slot to determine the actual size. Ensure that the filter you purchase is slightly smaller but no more than 1/2" smaller than this space when you look for filters online. In order for your filter to do its job properly, it's important that it fits as close to perfectly as it can. Any air that has the chance to escape unfiltered will, using the path of least resistance. On units without clips or brackets to hold in a filter, this can also result in annoying knocking or whistling sounds.
We include the actual and nominal sizes of filters to make buying filters easier.