Stitches per inch is the number buyers quote most often and understand least. Higher is not automatically better, and the right figure depends on what the seam has to survive.
What the number means
SPI counts the stitches in one inch of seam. At 8 SPI the needle penetrates the fabric eight times per inch; at 12 SPI, twelve. Every penetration is a small hole. That is the trade-off in one sentence: more stitches spread the load across more points, and also perforate the fabric more.
Where the density comes from
On coated and laminated fabrics, a high SPI can perforate the coating enough to start a tear along the stitch line — the seam becomes the weakest part of the panel rather than the strongest. On soft leather the same effect shows up as a cut line after the bag has been loaded a few dozen times.
For bag work, 7 to 9 SPI covers most structural seams in woven synthetics. Visible topstitching on leather goods often runs finer, 9 to 10, because it is doing as much aesthetic work as structural work. Webbing joins and strap anchors are not solved with density at all — they are solved with a bar-tack.
What to specify instead of “strong stitching”
- SPI for structural seams and, separately, for visible topstitching
- Thread type and tex size, not just colour
- Bar-tack positions — every strap anchor, handle join and zipper end
- Seam allowance, and whether internal seams are bound or taped
Those four lines in a tech pack remove more downstream argument than any adjective.