Card and Slot Sizes: The Formula I Use When Drafting Patterns

My first leather piece was a moneyclip with 3 card slots. At the time I did not calculate anything — I just saw that a slot was a little wider than the card, then measured, cut and stitched. The result: all 3 slots were tight; even sliding one card in meant wiggling it back and forth to fit, and taking it out was harder still.

Only after that did I understand that the problem was not thick leather or crooked stitching — it was that I had measured exactly the card size, without adding any allowance for the stitch line and the thickness of the overlapping leather.

Many people think all cards are the same size. In fact there is no international body that sets a common standard for business cards — sizes vary by country and culture. Bank cards and membership cards following the ISO/IEC 7810 ID-1 standard are fixed worldwide (85.60mm × 53.98mm), but personal business cards follow each country’s convention — in Vietnam 90mm × 55mm is common, while Japan uses 91mm × 55mm. Below is a table of common sizes in a number of countries for reference when needed:

But the numbers in the table are only a starting point. From the standard card size, I calculate the slot size on the pattern with this formula: +7mm on each side that has a stitch line.

Example: a standard ISO ATM card is 85mm × 54mm. The horizontal side is stitched on both ends: 7 + 85 + 7 = 99mm. The 54mm height stays the same — this side is not stitched, it just sticks up so you can take the card out.

If you use a 3mm-pitch diamond chisel, you need to round the size so it matches the exact number of punched holes. The formula I use: take a number divisible by 3, minus 1. For example, 99 is divisible by 3, minus 1 gives 98mm — that is the real size that goes on the pattern.

There is one more thing I once overlooked: when card slots are stacked on top of each other — like the 3 slots on that first moneyclip — every layer of leather between the slots also adds to the thickness. A single slot calculated on its own may fit snugly, but when you put cards into all the slots at once, the thickness of the stacked leather really shows — the whole block is much thicker than when you test each slot one by one. That was exactly when my moneyclip became hardest to use.

I still keep that first moneyclip, and now and then I take it out to look at it again. Stitch allowance, punching pitch and the thickness of stacked leather layers — those three things were what I lacked the first time.