Density is defined as how thick or thin the hair on your lace wig will be. The standard and most popular choice is 120% which is Medium Thick Density. The average human head is considered to be between 100%-120%.
Naturally growing hair grows differently for every person. Hair can grow thin (light) around the perimeter (hairline) and thick (medium) in the middle. Hair can also grow thick (medium) around the perimeter (hairline) and thickest (heavy) in the middle. Below is a Hair density Chart that you can use as a visual guide.