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Pricking iron spacing chart: mm to stitches per inch
The six common pricking iron spacings, from 2.7 to 5 mm, with stitches per inch (SPI), stitches per 10 cm, and the leather and thread each one suits.
Iron spacing in mm, SPI and stitches per 10 cm
| Iron spacing | Stitches per inch (SPI) | Stitches per 10 cm | Stitchline from the edge |
|---|---|---|---|
| 2.7 mm | 9.4 | 37 | 3 mm (≈1/8 in) |
| 3 mm | 8.5 | 33 | 3 mm (≈1/8 in) |
| 3.38 mm | 7.5 | 30 | 3.5 mm (≈9/64 in) |
| 3.85 mm | 6.6 | 26 | 3.5 mm (≈9/64 in) |
| 4 mm | 6.4 | 25 | 4 mm (≈5/32 in) |
| 5 mm | 5.1 | 20 | 4 mm (≈5/32 in) |
How to convert: stitches per inch = 25.4 ÷ the spacing in mm. A 3.85 mm iron gives 25.4 ÷ 3.85 ≈ 6.6 SPI. The other way round, spacing in mm = 25.4 ÷ SPI, so "7 SPI" is about 3.6 mm.
The stitchline from the edge is what Aptus patterns use for each spacing: wider spacing, slightly more distance from the edge, so the stitching looks balanced.
Which spacing for which project
| Iron spacing | Leather thickness | Thread thickness | Typical use |
|---|---|---|---|
| 2.7 mm | 0.8–1.0 mm (2–2.5 oz) | 0.30–0.40 mm | Watch straps, very fine work |
| 3 mm | 1.0–1.2 mm (2.5–3 oz) | 0.30–0.40 mm | Slim wallets |
| 3.38 mm | 1.2–1.4 mm (3–3.5 oz) | 0.45–0.55 mm | Most wallets and card holders |
| 3.85 mm | 1.2–1.4 mm (3–3.5 oz) | 0.45–0.55 mm | Most wallets and card holders |
| 4 mm | 1.4–1.6 mm (3.5–4 oz) | 0.60–0.80 mm | Bags, covers |
| 5 mm | 1.8–2.2 mm (4.5–5.5 oz) | 0.80–1.00 mm | Belts, sheaths |
These are typical ranges, not rules. Where pockets stack up to three or four layers, choose the thinner end so the piece stays slim: four layers of 1.4 mm already make a 5.6 mm edge.
Why the pattern has to match your spacing
Two spacings that look almost the same drift apart along an edge. Take a 100 mm edge drawn for 4 mm irons: it holds 25 stitches. Punch it with 3.85 mm irons and those 25 stitches cover only 96.25 mm, so the last hole lands 3.75 mm short of the corner — almost a whole stitch. You end up squeezing or stretching stitches at the corners, where it shows most.
That is why every Aptus pattern is drawn separately for each of the six spacings: the panel lengths and corners are worked out so the holes land where your iron punches. You choose the spacings you own when you buy; the price stays the same.
