Why a 16x25x1 Furnace Filter Isn't Actually 16 by 25 Inches

A typical furnace filter labeled 16x25x1 measures roughly 15.5 by 24.5 inches and three-quarters of an inch thick, so the printed size is a rounded-up nominal value. The HVAC Fitter team explains that the first two numbers can be swapped without changing the filter, and that unlabeled filters should be measured and rounded up to the next whole inch. Filter depths come in 1, 2, 4 and 5 inches, and a filter of the wrong depth should not be forced into the slot. The article provides a Python model that normalizes differently formatted size strings and checks whether one filter can safely replace another. The guidance is general information and does not replace a heating system's manual.