Two floor divisions, not one division
The arithmetic behind a strap count is not area over area. It is how many strips fit across the width, multiplied by how many straps fit along a strip, and both of those are whole numbers with a remainder thrown away. Take the rectangle in the form — 54 by 20 inches — and a 1-1/2 inch strap with a 1/16 inch trim between strips. The 20 inch width takes twelve strips of 1-1/2 with a 1/16 trim between them, leaving 1-5/16 inches over. A 48 inch strap out of a 53 inch strip after end trim gives one strap per strip and five inches wasted. Twelve straps, and five inches times twelve is five feet of leather in the bin.
Shorten the strap to 26 inches and each strip gives two, so the count doubles to twenty-four and the waste per strip drops to one inch. Nothing about the hide changed. The step is entirely in where the remainder lands.
The tag footage is measured round the outline
A tannery measures a hide by tracing its outline, which counts the neck, the two front leg flares, the belly and the tail, plus any brand or scar sitting inside the outline. That is a fair way to price a skin and a useless way to plan a strap. The 22 square feet on the ticket and the 7.5 square feet in the 54 by 20 rectangle are both true. The page prints the gap so you stop being surprised by it, and prints cost per strap against the tag footage rather than against the rectangle, because the tag footage is what left your account.
Orientation is not settled by the longer side
It is tempting to assume you should always run strips down the longer dimension. Sometimes that is worse. A 40 by 30 inch rectangle cutting 28 inch straps 2 inches wide: run the strips along the 40 and you get fourteen strips of one strap each, fourteen straps. Run them across and you get nineteen strips, each 39 inches long after trim, one strap each — nineteen straps. The short direction won by five, because 40 inches wastes eleven inches per strip and the extra strips more than pay for it. Both counts are printed, every time, for exactly this reason.
What the page cannot see
It sees a rectangle. A hide is not a rectangle, and the part of it you chalked is not uniform either — the leather nearer the belly is looser and stretchier than the leather over the backbone, and a strap that crosses that boundary changes character halfway along its length. Nor does the page know about a scar, a fly bite or a thin patch sitting in the middle of your rectangle, which turns one strip into two short offcuts. If your hide has one, chalk two smaller rectangles and run the page twice.
Questions people ask
How many belts can I get from one hide?
That depends on the rectangle you can chalk on the back, not on the square footage. A side tagged at 22 square feet might give a 54 by 20 inch clean rectangle, and out of that a 48 inch belt blank 1-1/2 inches wide comes twelve times — twelve strips, one blank each, with five inches wasted off the end of every strip. Change the blank to 44 inches and nothing changes; change it to 26 and you get twenty-four.
Why does the calculator ask for a rectangle instead of square feet?
Because a strap needs a straight run and square footage does not promise one. The footage on the ticket is traced round the whole outline including the neck, the belly and the tail. Feeding it a rectangle you measured yourself is the only way to make the answer match what comes off the bench, and it also makes the shape waste visible instead of hiding it inside a percentage.
Should strips run along the hide or across it?
The page counts both and tells you which gives more, because the answer flips depending on the numbers. Cutting 28 inch straps out of a 40 by 30 rectangle, running across gives nineteen and running along gives fourteen. Count is not the only consideration though — leather stretches more across the backbone than along it, and that is a decision about the strap rather than about the yield.
What should I put for trim between strips?
Whatever your cutter or knife actually removes, plus anything you skim afterwards to clean up the cut edge. A draw gauge or a strap cutter with a sharp blade takes very little, so a sixteenth is a common placeholder — but it is a placeholder, and the way to replace it is to cut four strips off a scrap and measure what is missing.
The strap will not fit. Does a bigger hide help?
Only if it is longer, and hides are not sold by length. If a 48 inch blank does not come out of your rectangle, a second hide of the same kind will not come out either — the shortfall is in one dimension and buying more area does not fix it. A back, a bend or strap leather sold by the running foot are all longer in the direction that matters.