Fabric width and usable width
A bolt can be sold as 44 inches wide and still be planned at 42 usable inches. The calculators ask for the width you are willing to cut, not the number printed on the end of the bolt.
Nominal width is not the cutting width
Quilting cotton is often sold around a 44 inch nominal width. Selvages, a printed edge, or a slight skew can make the usable portion narrower. A practical quilting plan often uses 42 inches.
Home-decor and drapery cloth is often planned around 54 inches, with 48 and 60 inch goods also common. Cushion work sometimes uses 46, 54, or 60 inches. Wide backing may be planned at 108 inches.
What to enter
Measure across the cloth you can actually use, from the inner edge of one selvage to the other, and subtract anything you will not cut into. Enter that number as usable fabric width.
Preset buttons fill a suggested width. They do not change the formula, and they do not claim that every bolt of that nominal width has that many usable inches.