Perimeter is only the starting number
Twice the quilt width plus twice the quilt length gives the exact perimeter. Binding needs more: every diagonal join consumes length, and the two ends need enough overlap to meet cleanly.
Yard & Loop adds a visible default allowance rather than hiding it inside the formula. Increase it for unfamiliar joining methods or very large projects.
Use the width you can really cut
Remove selvages before treating width-of-fabric as strip length. The first strip contributes its full usable width; each additional diagonal-joined strip contributes approximately the usable width minus one cut-strip width. Round up to a whole strip, then multiply strip count by cut width to get the fabric consumed across the grain.
A narrow remnant may hold enough square inches but still fail to provide the number of continuous full-width strips the plan expects.
Bias is a different plan
Straight-grain binding suits straight quilt edges. Curves and scallops usually need bias behavior, which changes the cutting layout and yardage. Do not use a straight-strip result as a bias-cut purchasing number.
Sources & further reading