Stitches Per Inch

The number of stitches per inch in your seam affects the strength and flexibility, and the SPI for lightweight fabrics will be different than the SPI for medium and heavyweight fabrics. Suzy recommends for lightweight fabrics, such as sheer fabrics, organza, and lightweight cottons, a higher SPI of 13-14 stitches per inch.

For mediumweight fabrics, use 11-12 SPI and for heavyweight fabrics such as wool and denim, use 10-11 SPI. The higher the SPI, the tighter and less flexible the seam will feel. For example, seams at the knee or elbow in thicker fabrics will feel stiff and uncomfortable if using an SPI that is too high. Sheer or lightweight fabric stitched with an SPI too low will be too weak.

The stitch length settings on each machine may be different, so Suzy recommends creating an SPI sampler to determine how many stitches per inch each stitch length setting on your sewing machine creates.

On a piece of sturdy fabric, mark parallel lines 1” apart. Then mark perpendicular lines at least ¾” apart to sew over using the different stitch length settings on your machine. Count how many stitches there are between the 1” lines for each stitch setting to determine the SPI for each, using a magnifying glass for the smaller settings if necessary.

For more sewing tips from professional sewing instructor Suzy Furrer, check out these videos:
Seven Essential Cutting Tools
How to Make Pocket Templates
When to Use Horizontal vs Vertical Buttonholes