Saddle stitch calculator, how much thread?
Seam length, stitch spacing and leather thickness in; the number of stitches and the length of thread to cut out, with the spacing converted between millimetres and stitches per inch.
The seam
How the length is worked out
In a saddle stitch the thread runs along both faces of the seam and through every hole twice, once from each side. So the thread has to cover 2 × seam length for the two faces, plus 2 × thickness for every stitch where it passes through the leather, plus the ends you hold. The calculator adds a quarter again for the tension you pull into each stitch and the little that is lost to twisting, and 30 cm for the needle ends.
That lands on the old rules of thumb: about four times the seam for thin leather and five times for thick. The rule breaks down on very short seams, where the needle ends are most of the thread, which is why a cardholder seems to use absurdly much.
Spacing and irons
- Pricking irons are sold by spacing in millimetres; stitching chisels often by stitches per inch.
spi = 25.4 / mm: a 4 mm iron is about 6.4 stitches per inch, a 3 mm iron about 8.5. - Fine goods (wallets, watch straps) use 2.7 to 3.38 mm. Bags and belts use 3.85 to 4 mm. Heavy straps and saddlery, 5 mm.
- Thread size follows the spacing: 0.45 mm thread for 3 mm irons, 0.6 mm for 4 mm, 0.8 mm for 5 mm is the usual pairing.
Leather thickness in ounces converts at 1 oz to 0.4 mm; the thickness converter does it for you.