Determining the Dimensions of your Filter
Most filters have two sets of dimensions: Nominal and Actual. A filter that has actual dimensions of 15½" x 24¾" x 1½" would be sold based on the nominal dimensions: 16" x 25" x 1".
When determining the size of filter you need, it is best to measure each dimension of your existing filter, and then find the closest nominal match.
When measuring your filter, the height & width are interchangeable:
16" x 25" x 1" is the same as 25" x 16" x 1"
You want your filter to fit snugly, without having to force it into place (this can cause the filter to tear or buckle, which could destroy the filter, or reduce its effectiveness). If, after installation, you find that there is air leaking around your filter, you can add gasket tape around the edges of your filter to fill in the gaps.