Density is the distance between individual stitches in a satin column or rows of stitches in a fill. The closer the rows are to each other, the denser an area of stitches is. Densities only apply to satin and fill stitch objects and underlay. With satin stitches, the narrower columns have less density than wider ones. Sample densities for underlay may be between 2 and 4 mm. Running stitches do not have density.
There are two primary ways that embroidery and digitizing systems measure density. One measures the actual space between rows (technically, between alternating rows, not each row) and the second is by counting the stitches per inch (SPI). Most embroidery software uses one of these systems. The first system may measure in metric or inches. Due to the small distances, metric is much easier to work with and may offer increments in millimeters or points, which are a tenth of a millimeter.
Because we are measuring space between stitches, the smaller the number the more dense the stitching. A reasonably average value for density is .4 to .45. A density of .2 is twice as dense as .4 and should probably never be used (with the possible exception of 3D foam) because it is simply cramming too many stitches into a small space.
In a well-digitized artful embroidery design, you will often find that density varies for two main reasons: interest and purpose. Light fills make great backgrounds, skies, and water allowing the eye to focus on the main subject, creating depth and perspective, and permitting the design to more gradually transition into the fabric, thus avoiding the “patch” look. Light densities provide less coverage. This can be a good thing when creating shadows, shading, building up layers of texture, or tone-on-tone embroidery. Lower density designs sew faster and stress the fabric less. The result is softer, more flexible embroidery.
High densities, especially when combined with short stitches, contribute to stiff, thick-feeling designs. When densities are excessive, you can experience increased thread breaks, broken needles, fabric damage, design distortion, and longer sewing times. Too much density combined with overly short stitches is one of most common mistakes made by novice digitizers and can be found in many of the free designs shared by them.
While you may not have fine control over stock embroidery designs, recognizing the impact of density can help you understand why some designs work better than others under different fabric/design/thread/stabilizer combinations. Keep in mind high density is not the only cause of a thick design; thread and stabilizer choices also affect the softness of embroidery digitizing.