Stitch density is the number of stitches in a given length of seam, usually given as stitches per inch (SPI) or stitches per centimeter (SPC). It is set on the machine by the feed length and is one of the main specifications on a garment tech pack.
Counting stitch density#
- Mark 1 inch (or 3 cm) on the seam, away from the start and end.
- Count the stitches within the marked length. For lockstitch, count the thread penetrations on one side; for chainstitch and overlock, count the needle penetrations.
- For SPC, divide the count by the length in centimeters.
Count at several places, because stitch length can change at seam crossings and curves.
Effects of stitch density#
| Too low (long stitches) | Too high (short stitches) |
|---|---|
| Weaker seam | Stronger seam up to a point |
| Seam grin: stitches open under stress | Needle damage and holes in fine or dense fabrics |
| Loose, untidy look | Puckering, especially on tightly woven fabrics |
| Less thread used | More thread and slower sewing |
| Stitches can snag | Fabric weakened along the seam line (perforation effect) |
Seam strength generally increases with stitch density until the needle holes start to weaken the fabric. The best SPI is a balance between strength, appearance and fabric damage.
Typical ranges#
These are common starting points, not requirements. The buyer's specification decides.
| Application | Typical SPI |
|---|---|
| Woven shirts, main seams (301) | 12 to 16 |
| Woven topstitching | 8 to 12 |
| Denim seams (401, 516) | 8 to 12 |
| Knit overlock seams (504, 514) | 10 to 14 |
| Knit coverstitch hems (406) | 10 to 14 |
| Fine lingerie | 14 to 18 |
Choosing stitch density#
Consider:
- Fabric: fine, dense fabrics pucker and get damaged at high SPI; loose fabrics need enough SPI to hold.
- Seam stress: seams that take load in wear (crotch, side seams on fitted garments) need adequate density.
- Appearance: topstitching often uses a longer stitch for a bolder look.
- Stretch: in chainstitch and overlock seams, higher density gives more thread in the seam and more stretch.
Stitch density and thread consumption#
More stitches per inch means more thread per seam. When SPI changes, the thread consumption ratio changes too. Measure ratios at the production SPI. See thread consumption.
Common problems#
- SPI changing across the seam: feed problems, presser foot pressure, or operator pulling the fabric. See uneven stitch.
- SPI set correctly but seam puckers: combine a lower SPI with correct thread tension and a finer needle. See seam puckering.
- Needle holes: reduce SPI, use a finer needle with the right point. See needle cut.
For seam strength testing, see seam strength.
Checking SPI on the line#
Stitch density should be checked:
- At machine setup for each style, against the tech pack.
- After any machine repair or adjustment.
- During inline inspection, on sampled garments.
- At final inspection, where the buyer's checklist includes it.
Keep a small SPI gauge or ruler at each line. Record the checks with the inline inspection results so that drift is caught early.
Related guides#
- Stitch types: the stitch types SPI is set for.
- Sewing thread selection: choosing thread strength along with stitch density.
- Sewing defects: defects caused by wrong stitch density.
- Sewing needle selection: needle size and point, which interact with SPI on fine fabrics.
Frequently asked questions#
Is SPI counted the same way for every stitch? Count needle penetrations in the marked length. For lockstitch, one stitch equals one penetration. For multi-needle stitches, count the penetrations of one needle.
Can I convert SPI to stitch length? Yes. Stitch length in mm = 25.4 / SPI. For 12 SPI, stitch length is about 2.1 mm.
Should topstitching and seams use the same SPI? Often not. Topstitching is sometimes longer for a bolder look, and seams under stress need adequate density. Follow the buyer's specification for each operation.