SPI stands for Stitches Per Inch. It measures how many stitch holes (or stitches) fall within one linear inch of a seam. In leather crafting, SPI is the standard way to describe stitch density. A higher SPI means finer, closer stitches (elegant look for small goods), while a lower SPI means chunkier, wider stitches (ideal for heavy-duty items like belts and saddlery). Most leatherworkers also refer to pricking iron spacing measured in millimeters – this tool helps you convert between the two systems seamlessly.

The right SPI depends on three main factors: project type, leather thickness, and aesthetic preference. For wallets and card holders, 7–9 SPI (3.0–3.6 mm spacing) is standard. Belts typically use 5–7 SPI for a rugged look. Fine watch straps can go up to 10–14 SPI for a luxurious finish. Thicker leather generally calls for lower SPI (wider spacing) so stitches don't tear through. Use our project selector above to get a recommended starting range, then fine-tune to your taste.

SPI (Stitches Per Inch) is the thread count per inch – it's the final result. Pricking iron spacing is the distance between the teeth of the tool used to mark holes, typically measured in millimeters. A 3.0 mm pricking iron produces roughly 8.5 SPI (25.4 ÷ 3.0 ≈ 8.5). They describe the same thing from different angles: SPI is the imperial/US convention, while millimeter spacing is the metric/tool convention. Most premium pricking irons (Vergez Blanchard, KS Blade, Sinabroks, etc.) are labeled by mm spacing.

Thicker leather requires wider stitch spacing (lower SPI) because stitches too close together can create perforation lines that weaken the leather – like a tear-off stamp. As a rule of thumb: for leather under 2 mm thick, you can use 8–12 SPI; for 2–4 mm leather, 6–8 SPI works well; for 4 mm+ thick leather, stick to 4–6 SPI. The stitch line should ideally be at least 3 mm from the edge, and stitch spacing should not be tighter than about 1.5× the leather thickness.

Thread size and SPI go hand in hand. For 10–14 SPI, use fine thread (0.4–0.6 mm, #92–#138). For 7–9 SPI, medium thread (0.6–0.8 mm, #138–#207) is ideal. For 4–6 SPI, heavier thread (0.8–1.2 mm, #207–#415) looks proportional. A good rule: the thread should fill about 60–80% of the stitch hole. Too-thin thread in a large hole looks sloppy; too-thick thread in a tiny hole won't pull through cleanly. Linen thread (Fil au Chinois, Maine Thread) and polyester (Ritza Tiger) are popular choices.

The formula is simple: Spacing (mm) = 25.4 ÷ SPI. For example, 8 SPI = 25.4 ÷ 8 = 3.175 mm spacing. Conversely, SPI = 25.4 ÷ Spacing (mm). A 3.0 mm pricking iron gives 25.4 ÷ 3.0 ≈ 8.5 SPI. Use our interactive tool above – adjust the slider and see both values update instantly. The preview canvas also shows you exactly what the stitch spacing looks like at a magnified scale.

Most professional leather crafters keep a few go-to spacings. 3.0 mm (≈8.5 SPI) and 3.38 mm (≈7.5 SPI) are the most versatile for wallets and bags. 3.85 mm (≈6.6 SPI) is popular for totes and larger bags. 4.0–5.0 mm (≈5–6 SPI) is standard for belts. Luxury ateliers like Hermès use very fine spacing (2.7–3.0 mm, or 8.5–9.4 SPI) for their iconic saddle-stitched goods. Many makers own 2–3 pricking irons at different spacings and choose based on the project.

Yes, but intentionally. Some advanced leatherworkers use tighter SPI on visible exterior seams and slightly wider SPI on interior pockets or less visible areas. However, mixing SPI on the same seam line is not recommended – it will look inconsistent. If you switch SPI, do it at a natural transition point (like moving from the main body to a gusset). For beginners, sticking to one consistent SPI throughout a project yields the cleanest results.

Not necessarily. While more stitches per inch distribute tension across more points, too-high SPI on thick leather can weaken the material by creating too many perforations close together (the "postage stamp effect"). For strength, match SPI to leather thickness. A saddle stitch (the gold standard in leatherwork) is inherently strong because each stitch is independently locked – even if one stitch breaks, the whole seam won't unravel. Proper SPI ensures the leather remains structurally sound while looking beautiful.

Quality pricking irons are available from brands like Vergez Blanchard (France), KS Blade Punch (South Korea), Sinabroks, Wuta, Kevin Lee, and Crimson Hides. They typically offer spacings from 2.7 mm up to 6.0 mm. European-style irons have a finer profile and create angled holes ideal for saddle stitching. Japanese-style chisels (diamond shape) are more affordable and beginner-friendly. Choose based on your budget and the SPI range you use most – a 3.0 mm or 3.38 mm iron is a great all-rounder to start with.