Replacing your air conditioner filter in a timely manner is much simpler when you know the size of the filter you need. Air filters come in a variety of sizes, so it can be confusing if you don't know what size to get. If you have to force the filter in or out, it's not the right size - usually, standard sizes are half an inch smaller than advertised. The Lennox filter below (see image) shows part number X0583 and Honeywell part numbers FC100A1029 and FC35A1001, which this filter can replace.
Most pleated and white filters are generally the same within each efficiency level and may vary slightly between brands. If the filter is not the right size, either too small or too large, air will flow around the filter instead of through it, meaning not all air will be filtered. If the dimensions are too faded to read or there's no filter to look at, you'll need to measure it yourself. Round down to the nearest whole number to determine the nominal size of the air filter.
Filterbuy states that 70% of their sales don't exceed the 10 most common air filter sizes, which can usually be found in large stores such as Home Depot, Walmart and Lowe's. This video provides a guide on how to identify the right air filter for your HVAC system.