How to Measure the Size of an Air Filter

Measuring the size of an air filter is essential for ensuring that your HVAC system runs efficiently and safely. If the filter is not the correct size, either too small or too large, air will flow around the filter instead of through it and, as a result, not all of the air will be filtered out. Worse, improperly sized air filters can put excessive strain on your HVAC system, damage it, and increase your energy bills. The easiest way to identify the size of the AC filter is to remove the existing filter from its slot and examine the filter frame.

Typically, the filter size is printed along the edge of the filter frame. Note that the size shown in bold type on the filter frame is the nominal size of the filter, which is likely to differ from its actual size. If the actual dimensions do not appear in small print on the filter frame, we recommend measuring the filter for double checking. If you're looking for a retailer near you that sells custom-sized air filters, you might struggle to get lucky. When in doubt, you can always measure your air filter with a ruler or any other measuring device.

Here's how to do it:Step 1: Measure the length and width (L x W). If the filter is not square, the smallest measurement goes first. Step 2: Measure the depth (D).Then use a measuring tape or other measuring device to measure the length, width, and depth of the inner edge of the groove or frame of the air cleaner. When buying air filters, you will see the words “nominal size” and “actual size” together. In some cases, the length and width are interchangeable, but you'll always need to make sure to measure those 3 specific dimensions of air filter or oven filter. Watch this video for step-by-step instructions to find out how to obtain both your actual and nominal sizes:.