Standard air filter sizes may vary slightly, and if you have to force an air filter in or out, it's not the right size. Common filter dimensions are usually half an inch smaller than advertised. For example, a filter labeled 10 X 12 X 1 measures 10 inches long, 12 inches wide, and one inch thick. The first important difference between 1-inch and 2-inch filters is their lifespan.
Be aware of the thickness, as it will affect the next decision you have to make. Most residential HVAC filters are 1 inch thick, but 4-inch filters are also becoming common. Your HVAC professional will install a small cabinet next to the boiler or air controller (on the air intake side) to hold the thicker filter. The gaps will allow dust, dirt and germs to bypass the filter and potentially re-circulate to the air in your home.
When measuring wood or air filters, it's important to note that the nominal size is not the same as the actual size. The filter should be slightly smaller on each side of the slot in the filter housing, allowing you to slide the filter in and out with ease. On the other hand, if the filter is too large, the frame will wrinkle, air flow could be restricted and longevity would be compromised. At MERV 13, the highest rating you'll find for most popular residential HVAC filter sizes.
You can trust that the filter will stop bacteria, smoke and other microscopic particles. Filters don't need to fit a “hermetic seal”, but you should be able to easily install and uninstall the filter without problems. An easy way to choose a replacement filter is to simply buy the same filter that you already use in your system. Filters should be changed every 30 to 90 days, depending on the type and efficiency of the filter.
If the filter is too small, it will be loose and some of the air (and dirt) will not pass through the air filter. However, it should fit snugly in the air cleaner compartment with a little space around the edges. Air filters generally come in a variety of standard sizes, and some can be adapted to fit different sizes of filter boxes or air return openings.
