The margins are fixed and the pitch is not
Ask for rivets every 1-1/2 inches along a 9 inch run with half inch margins and you are asking for the impossible. The span between the first and last rivet is 8 inches, and 8 does not divide by 1.5. The page rounds to five gaps and six rivets, which lands the pitch at 1-19/32 inches — 3/32 wider than asked. The alternative is to hold 1-1/2 exactly, which leaves the last rivet half an inch short of where the margin wanted it, and that is the error people see from across a room.
Five rivets on the same run is the tidier answer: four gaps of exactly 2 inches, margins held, nothing rounded. Whole numbers of gaps are worth chasing when the run allows it.
Edge distance eats the width twice on two rows
A quarter inch edge distance on a 1-1/2 inch strap leaves the two rows 1 inch apart, which is comfortable. Take the strap down to 3/4 inch and the same edge distance leaves the rows 1/4 inch apart, centre to centre — and with a 0.12 inch post that is 0.13 inches of leather between the two holes. The page prints that figure rather than a rule about it, because how much leather has to sit between two holes is a decision about the leather, the load and the look, not something a calculator gets to declare.
The other figure worth watching is the web between the hole and the edge: edge distance less half the post diameter. At a quarter inch edge distance and a 0.12 post that is 0.19 inches. Drop the edge distance to a sixteenth and it goes negative, which means the punch breaks out through the edge.
What the post has to bridge
Two layers of 8 oz and 5 oz glued together is 13 oz, which is 13/64 or 0.203 inches. A quarter inch post leaves 0.047 inches standing above the leather for the cap to take up. Whether that is the right amount is a question for the rivet packet — a double cap rivet, a rapid rivet and a copper rivet over a burr all want different things, and none of them are stated here as fact. The arithmetic is post length minus stack, and it is printed because it is the number people forget to do before ordering three hundred rivets.
What it does not know
It does not know whether the rivets will hold, how the leather will behave round the holes after a year, or whether your press will set that post cleanly through that stack. It also assumes a straight run. A rivet line following a curve has a different arc length from the chord you measured, and the way to handle that is to run a tape along the curve and feed the page the tape reading.
Questions people ask
How far apart should rivets be on a strap?
The page will not tell you that, because it depends on the strap, the load, the leather and how it should look — and a calculator has no business declaring any of those. What it does is take the spacing you want and show you what actually lands: with half inch margins on a 9 inch run, five rivets fall exactly 2 inches apart, and asking for 1-1/2 inch spacing gives you six rivets at 1-19/32 instead.
Why is the pitch different from what I asked for?
Because a whole number of gaps has to fit between the first and last rivet, and the span rarely divides evenly. The page holds both end margins and lets the pitch move, which is the version that looks deliberate on a finished strap. Holding the pitch instead would push one end margin out by the accumulated difference, which on five gaps is five times the per-gap error — half an inch, when each gap is out by 3/32.
What edge distance should I use for rivets in leather?
Your own figure, and the page prints its consequences rather than proposing a number. What it shows is the web left between the hole and the edge — edge distance minus half the post diameter — and, on two rows, the leather left between the two holes. A quarter inch edge distance with a 0.12 inch post leaves 0.19 inches to the edge on a 1-1/2 inch strap.
How long a rivet post do I need?
Add up everything the post passes through and compare it to the post you have. Two layers of 8 oz and 5 oz is 13 oz, which is 0.203 inches, so a quarter inch post stands 0.047 inches proud. Whether that is the right amount of proud is set by the rivet type and by the packet it came in, and this page does not state it.
Does this work for snaps and Chicago screws?
The spacing part does — the positions along a run are the same arithmetic whatever goes in the hole. The through-the-stack part works for anything with a post you can measure, including a Chicago screw where the post is the barrel. What changes is the hole diameter and what the packet says about how much the hardware needs to take up, and both of those are fields rather than assumptions.