Stitches Per Inch on a Leather Jacket: What SPI Tells You About Quality

Stitches per inch (SPI) is the number of stitches in one inch (25.4 mm) of a stitch line. To get it, count the stitches in a measured inch, or divide 25.4 by the stitch length in millimeters. A 3 mm stitch is about 8.5 SPI.

There is no single "good" SPI for a leather jacket, and none of the sources we read gives a garment benchmark. Smaller stitches are generally regarded as finer work, but a stitch that is too small can make the leather tear along the seam. What the count really shows is whether the stitching is even, and whether the stitch size suits the leather. This page is part of the leathercraft and garment construction guide.

Convert stitch length to SPI

If you know the stitch length, the conversion is one division: 25.4 divided by the length in mm.

A home-sewing retailer suggests about 3.0 mm for seams and a longer stitch for topstitching, which works out to about 8.5 SPI for seams. That is advice for home machines, not a factory jacket standard.

How to count SPI on a jacket, step by step

You need a ruler or tape marked in inches and a pen-sized pointer. Use a seam you can see clearly, such as topstitching along a front edge, collar or pocket.

1. Pick a straight stretch. Counts on curves and corners mislead because stitches bunch or spread there.
2. Mark one inch. Lay the ruler along the stitch line and hold it flat.
3. Count whole stitches inside the inch. Count stitches, not holes. On a machine seam each stitch shows as one short dash on the surface.
4. Repeat in four or five places. Try a front edge, a sleeve seam, a pocket and the collar, and write each count down.
5. Convert if you prefer millimeters. Divide 25.4 by your SPI to get the stitch length in mm.
6. Compare the counts. Similar seam types on the same jacket should give similar counts. A jump from 8 to 5 along one seam is a sign worth noting.

What the count can and cannot tell you

Consistency. A store guide says topstitching should keep a consistent stitch length and straight lines. Even counts along a seam matter more than the number itself.

Fit to the leather. One UK jacket maker resets stitch size for each hide weight and uses a smaller stitch on thinner hides, but never one so small that the close needle holes let leather tear along the seam like perforated postage stamps. A store guide adds that every needle hole in leather is permanent.

Thinned seams. The same maker says a skived seam combined with a small stitch can become a weak point. Other sources describe skiving as removing bulk. See skiving in leather garment making for both sides.

What it cannot show. SPI says nothing alone about thread strength, needle choice or the leather itself. That maker uses a heavy-duty cotton thread with a polyester core and says it found pure cotton thread to be usually the weakest point on vintage jackets it restored. Store guides name industrial machines made for leather, with suitable needles and heavy or bonded thread.

Hand stitching: why SPI labels confuse

If you stitch by hand, tool sizes do not line up between brands. Forum members report that some brands size by prong width and others by the distance between points, so the same millimeter number can mean a different spacing. Check the maker's own chart, or use the formula above to confirm.

Hobbyist examples are project-specific. One member says 6 or 7 SPI with 0.8 or 1.0 mm thread works well for holsters, and another says no more than 6 for a first saddle; none of the posts addresses garments. For tools and the stitch itself, see leather stitching: stitches, tools and construction basics, common leather stitching types and how to hand-stitch leather.

Using SPI as one check among several

Count SPI alongside the rest of a jacket inspection. Hand-stitch slant, edge finish, lining and hardware also say something. Our guides to what to look for in a quality men's leather jacket and how to assess the quality of a leather motorcycle jacket cover the full check. For how seams are built, see how a leather jacket is made.

Leather Collection's guide to how leather jackets are made says seams, zippers and linings show quality most clearly in daily wear.

FAQ

How do you determine stitches per inch?

Count the stitches in a measured inch of a straight stitch line, or divide 25.4 by the stitch length in millimeters.

How far apart should stitches be?

No source we read gives a garment rule. A home-sewing retailer suggests about 3.0 mm for seams, about 8.5 SPI. A jacket maker adjusts stitch size to hide weight.

Is a higher SPI always better?

No. Smaller stitches are generally regarded as finer, but stitches too close together risk tearing along the seam.

What is the strongest thread for leather?

The sources we read do not rank thread. A UK jacket maker prefers a cotton-covered polyester-core thread and says pure cotton was usually the weak point on restored jackets.