Stitches Per Inch (SPI)

Count the stitches landing in one inch of a sewn seam and you've got the number that shows up on every cut-and-sew quality checklist: stitches per inch, or SPI. Lightweight knit tee side seams commonly run 8 to 10 SPI; heavier fleece and denim construction often drops to 6 to 8 SPI because thicker fabric needs a bigger stitch to avoid puckering, while stretch and performance knits can push past 10 SPI for recovery under pull.

A higher stitch count generally builds a stronger, more durable seam, while a lower one sews faster and lies flatter on heavy fabric. On a fleece hoodie's coverstitched hem — a Gildan 18500, say — we look for a steady 8-plus SPI; anything looser and the hem gaps open after a handful of wash cycles, especially once a heavy full-back print adds stiffness that fights the seam every time it flexes.

Inspectors use SPI as a fast pass-fail check: too few stitches per inch and a seam is prone to popping at stress points like the underarm or crotch; too many and the needle perforations can weaken the fabric along the line, an issue called needle-cutting. Pros spec an SPI range by garment type in the tech pack and confirm it on a pre-production sample before a bulk cut goes to the line.

It's easy to assume a higher SPI is simply better across the board, but that's not how it works — 12 SPI on a stretch waistband makes it rigid and prone to thread breakage under pull, while that same count is standard on a woven collar. The right number depends on the fabric and the seam's job, not a single universal target.