Stitches per inch by leather weight
Match your stitch spacing to the leather — fine on thin, bold on thick.
Stitches per inch (SPI) is set by your stitching irons or pricking irons, and it's one of the biggest drivers of how hand-stitched leather looks. The rule that ties it together: the heavier the leather and the larger the item, the lower the SPI; the thinner the leather and the finer the goods, the higher the SPI. A stitch that looks crisp on a card wallet looks toy-like on a belt.
SPI by leather weight and project
Leather thickness is measured in ounces (1 oz ≈ 1/64 inch). A practical starting guide:
| Project | Leather weight | Typical SPI |
|---|---|---|
| Wallets, card holders, fine goods | 2–4 oz | 7–9 |
| Bags, watch straps, medium goods | 4–7 oz | 6–7 |
| Belts, sheaths, heavy goods | 8–12 oz | 4–6 |
SPI to millimeters
Irons are often sold by spacing in millimeters, so it helps to convert (spacing = 25.4 ÷ SPI):
| SPI | Spacing |
|---|---|
| 9 | ~2.8 mm |
| 8 | ~3.2 mm |
| 7 | ~3.6 mm |
| 6 | ~4.2 mm |
| 5 | ~5.1 mm |
Proportion is the real goal. SPI isn't about strength so much as looking right for the piece. Thicker thread suits lower SPI and heavier leather; fine thread suits higher SPI. Match thread thickness, iron spacing, and leather weight as a set.
Packing many small holes close together in heavy leather perforates it like a stamp — the stitch line can tear. Lower SPI on thick leather isn't just aesthetic; it leaves enough material between holes.
Gear this guide uses
SPI is set by your irons — and matched to thread thickness and leather weight as a set.
- Choose stitching / pricking irons by their millimeter spacing to lock in your SPI.
- Pair them with waxed thread sized to the holes and your veg-tan leather weight.
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Sources
Weights and SPI are starting points — match thread, iron, and leather as a set for your piece.