Free tool
Pricking iron size chart
Pricking irons and stitching chisels are sold by the spacing between their prongs, in millimetres, or by stitches per inch (SPI). This chart puts common sizes side by side, and the converter turns any size into the others.
How to convert mm to stitches per inch
An inch is 25.4 mm. Divide 25.4 by the spacing in millimetres to get stitches per inch, and divide 25.4 by the stitches per inch to get the spacing back.
- 3.85 mm: 25.4 ÷ 3.85 = 6.6 SPI
- 3.38 mm: 25.4 ÷ 3.38 = 7.5 SPI
- 8 SPI: 25.4 ÷ 8 = 3.175 mm, which is 1/8″
To count the stitches on a seam, divide its length by the spacing: a 200 mm seam at 3.85 mm is 51.9 spacings. The stitch hole spacing calculator turns that into a whole number of holes and handles the corners.
How to measure an iron you already have
One spacing is too small to measure well, so measure across several. Press the iron into a scrap of leather or card, then measure from the first hole to the last with a steel rule and divide by the number of gaps. A 6-prong iron has 5 gaps: if its first and last holes are 19.25 mm apart, the spacing is 19.25 ÷ 5 = 3.85 mm. Measure from the centre of one hole to the centre of the other, or from the same edge of each.
Why size charts disagree
Two charts can give the same iron two different numbers. Three things cause most of it:
- Rounding. 3.85 mm is 6.6 SPI. Rounded up or down to a whole number, it can be listed as 6 or 7.
- Counting prongs. An inch of 8 SPI holes has 8 stitches and 9 holes. A chart that counts holes or prong points gives one more than the stitches.
- Size codes. Some irons carry a size number that isn't a measurement at all. Check the spacing in millimetres before you buy.
Which size for which project
There's no single right size. Smaller spacing gives a finer seam and suits thin leather and thin thread. Larger spacing suits thick leather, heavy thread and pieces that take strain. As a starting point:
- Wallets, card holders and small goods: 2.7 to 3.38 mm (7.5 to 9.4 SPI).
- Bags, belts and larger wallets: 3.85 to 4 mm (6.4 to 6.6 SPI).
- Belts, straps and heavy bags: 4.5 to 5 mm (5.1 to 5.6 SPI).
- Holsters, sheaths and thick straps: 5 to 6 mm (4.2 to 5.1 SPI).
Before you commit, prick a few rows on a scrap of the leather you'll use and sew them with the thread you'll use.
Diamond, French, flat and round
The spacing sets how many stitches fit. The prong shape sets how they look. Diamond prongs cut a slanted diamond, French prongs a thin slanted slit, flat prongs a straight slit and round prongs a round hole. The same spacing looks different in each shape, so try both on a scrap if you're choosing between a diamond and a French iron.
Your irons in PatinaCAD
PatinaCAD has diamond, French, flat and round irons built in, from 3 mm to 5 mm plus 1/8″ and 5/32″: the last column of the chart. If yours isn't there, add it with its exact spacing, blade size and prong counts in the pricking irons list, and PatinaCAD spaces the holes to match it.
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Add the irons on your bench to PatinaCAD, and every pattern gets holes spaced for the iron you'll actually use.