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Leather "ounce" weight refers to weight per square foot, which correlates to thickness. One ounce ≈ 0.4 mm in the standard conversion used by North American tanneries.
Thickness (mm) ≈ ounces × 0.4The iron (or ligne) is a traditional leathercraft unit. One iron equals 1/48 inch or approximately 0.529 mm, used widely in European leatherworking.
Thickness (mm) = irons × 0.529Convert between any two units by passing through millimeters as the common base. Round to practical fractions for skiving and machine settings.
oz ≈ mm / 0.4; irons ≈ mm / 0.529Updated: July 2026
4–5 oz ≈ 1.6–2.0 mm. Use 0.8–1.0 mm thread (Lin Cable 432 or Ritza 25) with 3.0–3.5 mm stitch holes for balanced hand stitching.
Pattern calls for 5 iron leather (~2.65 mm). Convert to 6–7 oz domestic equivalent when ordering from a US tannery listing ounce weight.
Clicker die rated for 3 mm max. A 9 oz strap at 3.6 mm exceeds capacity — split or skive, or select thinner stock before cutting.
A "5 iron" thickness (2.65 mm) is unrelated to a "5 mm pitch pricking iron" spacing tool. Thickness irons measure hide depth; pricking irons set stitch hole spacing.
Dense latigo and loose-pull-up leather at the same oz can differ 0.2–0.3 mm. Caliper your actual hide at multiple points before precision fitting.
Leather is sold and specified in ounces (oz), millimeters, or irons depending on region and trade. This converter translates between all three systems so you can match materials to patterns, machines, and thread recommendations.