What is Density?
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 more dense an area of stitches is.
How is Density Measured?
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), which I won't spend any more time discussing. Most embroidery software uses one of these systems, although there is one popular home software that uses an arbitrary scale that is relative, making it hard to be precise.
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.
Why Density Can Be Confusing
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. If the density is .8, it is half as dense as .4; the stitching is more open because there is twice as much space between the rows. This is a nice value for shading layers or lighter backgrounds like skies. 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.
Where is Density Used?
Density only applies to satin and fill stitch objects and underlay. With satin stitches, the narrower column have less density than wider ones. Sample densities for underlay may be between 2 and 4 mm. Running stitches do not have density.
How Density Affects Your Embroidery
In a well-digitized artful 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 a 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.
Note: Increasing density provides better coverage up to a point. For more better fabric coverage, lengthen the stitch, increase the underlay, or use a color-block topping.
The perfect combination of density and stitch length covers the fabric adequately without creating a "bullet-proof patch." Unfortunately, "perfect density" is relative and depends on the fabric, the color of the fabric relative to the design, and the embroiderer's personal preference. Always keep in mind that embroidery is an embellishment and absolute total solid coverage is not the goal and seldom results in a good embroidery.
How To Control Density
As an embroiderer, you will probably most likely be faced with density choices when using a lettering program. Avoid the temptation of dramatically increasing density on letters, especially when working with very small letters. The rule of thumb here is that the narrower the column (thinner the letter stroke) the less density you should apply.
If you are using stock designs, you probably have little, if any, control over initial density settings. There are some utility programs-I recommend Density Works from Designer Gallery-that can detect and correct areas of high density. Density Works is very easy to use and should be part of every embroiderer's tool box.
While you may not have fine control over stock 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 an embroidery.