If your old filter is printed “12x12x1,” the actual filter you need measures 11.5 x 11.5 x 0.75 inches. Manufacturers round the nominal (printed) size up to the nearest whole number for the box — the real filter is always slightly smaller so it slides into the frame.

Before you buy: pull your old filter and check the actual size printed in small text on the frame — most brands match 11.5 x 11.5 x 0.75″, but some product lines run up to ~0.4″ larger.