SPI — stitches per inch — is the measure of how dense the stitching is along a seam. It's a small number that has a real impact on seam quality: too few stitches and the seam is weak; too many and the seam becomes rigid, perforates the fabric, and loses elasticity. For jersey, 10–12 SPI is the calibrated standard that balances both.
Most factories will default to a production-optimized stitch density if you don't specify — often faster (fewer stitches per inch) to increase throughput. Specifying 10–12 SPI in your construction notes ensures the seam is built to the right quality standard, not the fastest possible setting.
Always include SPI in your construction notes. 10–12 SPI for all jersey seams — overlock and coverseam. It's a one-line spec that directly affects seam strength and durability.
How stitch density affects seam quality
Production specs for your tech pack
| Spec | Value | Notes |
|---|---|---|
| Stitch density | 10–12 SPI | Applies to all seam types |
| Overlock seams | 10–12 SPI | Side seams, shoulder, sleeve set |
| Coverseam hems | 10–12 SPI | Bottom hem, sleeve opening, neckband topstitch |
| Tolerance | ±1 SPI | Acceptable range: 9–13 SPI |
In FlatLabs PRO, 10–12 SPI is automatically included in construction notes for all jersey garments — alongside seam type, thread spec, and needle type. One complete construction block, no manual lookup.
SPI and seam elasticity
Jersey garments stretch. When you pull the side seam of a t-shirt, the fabric stretches — and the seam has to stretch with it. At 10–12 SPI, the overlock stitch has enough loop structure between stitches to accommodate that stretch without breaking. The stitch itself is elastic by design (that's the nature of overlock construction), and 10–12 SPI maintains that elasticity without compromising strength.
This is why SPI spec matters more for knits than for wovens. A woven shirt doesn't stretch much — SPI mainly affects strength. A jersey t-shirt stretches constantly — SPI affects both strength and whether the seam moves with the garment or fights it.