When it comes to air filters, size matters. The difference between a 20x20x4 air filter and a 20x20x1 filter is significant, and understanding the numbers can help you make the right choice for your home. The MERV (Minimum Efficiency Reporting Value) standard is used to measure the performance of filters intended for forced air HVAC systems. HEPA (High Efficiency Particulate Air) is a much more stringent standard than MERV, but it tends to restrict airflow too much for most residential forced air systems.
Instead, HEPA filters are commonly used in air purifiers and vacuums. If the filter does not have the actual dimensions on the outside of the filter, you can measure it to get the correct dimensions. A 4-inch filter will last longer and provide better air quality for your home than a 1-inch filter. That said, the 1-inch filter may be too shallow, causing a loose fit that allows unfiltered air to enter the ducts.
If you currently have 1-inch filters, you might consider upgrading your system to fit 4-inch filters. If the existing filters fit perfectly, note (or take a picture) of the size printed on the filter frame. You may need to replace the filter more often if you have some very furry pets, for example, or if you live in an area with a lot of air pollution (such as from wildfires). Like length and width, thickness is an important part of sizing oven filters and obtaining the right fit to optimize performance.
Wall or ceiling ventilation WITHOUT clips or brackets to secure the air filter in place requires a tight fit to avoid sounds of free movement, flapping, tapping or whistling that may come from an air filter that fits too loosely. Most air conditioning and HVAC units use standard size filters, but some require a custom-sized air filter to be purchased and installed. But these are usually just filters with a rating of at least MERV 11, which is when filters begin to trap most particles that are the size of common allergens. Some filter brands and retailers use alternative scales, such as Home Depot's Air Filter Performance Rating (FPR) system or the MPR (in 3M Filtrete air filters).