Pick a standard zipper size for an opening.
Standard zippers come in set lengths; the next size up can be shortened.
A zipper should be matched to the length of the opening, and the standard sizes come in set lengths, so the calculator picks the nearest size that fits. For most openings a zipper the length of the opening or a little longer works, and a longer zipper can be shortened by trimming and adding a new stop.
Invisible zippers are usually made a little longer than the opening, because the tape extends past the seam and the zipper pull needs room to open fully. Check the instructions on the package for the allowance that your type needs.
Choose the type and weight for the fabric and the use. Lightweight nylon coil zippers suit dresses and skirts, heavy metal zippers suit jackets and bags, and separating zippers suit coats that open completely. A zipper that is too heavy for the fabric will drag the seam and make it pucker.
Install the zipper before you add the waistband or facing, and test it a few times before you finish. Press the seam and the zipper tape with a low heat, since the tape and teeth can melt.