When replacing your air filter, most people will find the nominal size or dimensions located on the edge of your current air filter. If you don’t have the previous filter handy, you’ll need to measure your intake vent for the actual size of your air filter.
"Nominal" refers to the product sizing chart that fits into your intake vent. "Actual" size refers to the true measurements of your air filter dimensions, which will be slightly smaller than the intake vent dimensions. When searching for a 16x25x1 air filter, for example, you’ll likely see 15.5" x 24.5" x .75" listed. Nearly all AC filters go by the nominal size, which is the actual size rounded up to the nearest inch.