How to calculate or check machine SPI ( Stitches Per Inches)

The abbreviation of SPI is Stitches per inch. In the Metric System it is expressed as Stitch per centimeter (SPC). It is very easy to measure SPI of the machine or seam. To measure it, take a fabric swatch of 12 inch X 2 inch. Sew the fabric length wise in a single burst with current SPI setting. Initially use contrast thread for bobbin and needle thread. Take out the stitched fabric and lay on a flat table. Remove all creases if present on the seam line by hand. Now, take one measuring tape, place it on the stitch line on the above fabric sample and mark line 2 inches apart. See the following figure.

Now count number of total stitches in between those two lines. Divide total number of stitches by 2. The result is the SPI of that machine or seam. For SPC measurement is taken in centimeters. If your SPI requirement is less or more than the current machine SPI then rotate SPI regulator accordingly (for basic machines). After setting check SPI again in the similar method. When you get correct SPI on the sample then go ahead for production.

Example: In the above figure, the total count of the stitches is 20 (in 2 inch gap). So SPI will be 10.