Leather Stitches Per Inch (SPI) Calculator
Created by: Isabelle Clarke
Last updated:
Convert your pricking iron or stitching chisel spacing to stitches per inch (SPI) and stitches per cm. Enter a seam length to see total stitches, recommended thread weight, and estimated hand-sewing time.
Stitches Per Inch Calculator
LeathercraftConvert pricking iron spacing to SPI, stitches per cm, and stitch count
Pricking Iron / Stitching Chisel
Seam Details (Optional)
Enter seam length to calculate total stitches and estimated sewing time.
Related Calculators
What is a Leather Stitches Per Inch Calculator?
A Stitches Per Inch (SPI) Calculator converts pricking iron or stitching chisel spacing (in mm) to stitches per inch and stitches per centimetre. It tells you exactly how dense your stitching will be and helps you plan thread, time, and seam appearance for any leather project.
SPI directly affects the look, strength, and durability of your leather goods. Too few stitches per inch and seams look crude and may pull apart under stress. Too many and you perforate the leather, weakening the edge. This calculator helps you choose the right iron size for your project.
SPI Conversion Formulas
SPI: 25.4 mm ÷ Iron spacing (mm)
Stitches per cm: 10 mm ÷ Iron spacing (mm)
Total stitches: Seam length (inches) × SPI
Estimated time: Total stitches ÷ Stitches per minute (typically 3-5 for saddle stitch)
Pricking Iron SPI Reference
| Iron Spacing | SPI | Stitches/cm | Best For | Thread Size |
|---|---|---|---|---|
| 2.7mm | 9.4 | 3.7 | Ultra-fine work, watch straps | 0.35-0.45mm |
| 3.0mm | 8.5 | 3.3 | Fine goods, card holders | 0.45-0.55mm |
| 3.38mm | 7.5 | 3.0 | Wallets, small cases | 0.55-0.65mm |
| 3.85mm | 6.6 | 2.6 | General leatherwork, bags | 0.55-0.65mm |
| 4.0mm | 6.4 | 2.5 | Bags, totes, general | 0.6-0.8mm |
| 5.0mm | 5.1 | 2.0 | Heavy work, holsters | 0.8-1.0mm |
| 6.0mm | 4.2 | 1.7 | Belts, saddles, heavy straps | 1.0-1.2mm |
| 8.0mm | 3.2 | 1.3 | Saddles, horse tack | 1.0-1.4mm |
SPI Tips for Leather Workers
- Match thread weight to iron spacing — thick thread in small holes looks bulky and is hard to pull through.
- Practice SPI consistency on scrap leather before starting a project — uneven spacing looks amateur.
- European-style pricking irons create angled holes for a diagonal stitch line; Japanese-style create straighter holes.
- For curved seams, closer SPI (smaller iron) follows curves more smoothly with fewer angular jogs.
- Consider needle size too — a too-large needle widens holes and weakens the leather edge.
Frequently Asked Questions
What is SPI (Stitches Per Inch) in leathercraft?
SPI (Stitches Per Inch) measures stitch density — how many stitches fit in one inch of seam. Higher SPI means tighter, finer stitching. For leather, typical SPI ranges from 5-8 for general work, 8-10 for wallets and small goods, and 3-5 for heavy items like saddles and belts. SPI is determined by your pricking iron or stitching chisel spacing.
How do pricking iron sizes relate to SPI?
Pricking irons are labelled by tooth spacing in mm. A 4mm iron produces ~6.35 SPI (25.4mm per inch ÷ 4mm). A 3mm iron gives ~8.5 SPI. Common sizes: 3mm (8.5 SPI, fine work), 3.38mm (7.5 SPI, wallets), 3.85mm (6.6 SPI, general), 4mm (6.35 SPI, bags), 5mm (5.1 SPI, heavy work), 6mm (4.2 SPI, belts/saddles). This calculator converts any spacing to SPI instantly.
What SPI should I use for different leather projects?
Fine goods (card holders, watch straps): 7-10 SPI with 3-3.38mm irons. General leatherwork (wallets, pouches, bags): 5-7 SPI with 3.85-4mm irons. Heavy-duty items (belts, holsters, knife sheaths): 4-5 SPI with 5-6mm irons. Saddles and horse tack: 3-4 SPI with 6-8mm irons. Always match thread weight to SPI — finer stitching needs thinner thread.
What is the difference between pricking irons and stitching chisels?
Pricking irons create angled guide holes and are designed for hand saddle stitching — the holes are slanted so the thread sits at an angle for a professional look. Stitching chisels punch straight-through holes and are better for machine-like stitching or when using a machine later. Both are measured by tooth spacing, and SPI calculations work the same for both.
How do I calculate total stitches for a seam?
Multiply your seam length (in inches) by your SPI. For example, a 12-inch seam at 6 SPI = 72 stitches. For saddle stitching, each stitch requires two needle passes, so you make 144 needle passes total. This helps estimate sewing time — most hand stitchers average 3-5 stitches per minute for saddle stitching.
Sources and References
- Kevin Lee / KS Blade Punch, "Pricking Iron Spacing Guide", 2024
- Sinabroks / Crimson Hides, "Choosing the Right Pricking Iron", 2024
- r/Leathercraft Community, "SPI and Iron Spacing Discussions", 2024