Z-Density, which presumably stands for Zombie Density, is the measure of danger posed by the quantity of zombies in a zone. SCUGS represents this level of danger by displaying different colors and density in a percentage, 0% being the lowest and 100% the highest.

Z-Density does not represent the number of zombies in a zone directly, but rather the density of zombies per square. The more zombies in the zone, the higher the density.

Density is color-coded, as shown on the bottom-left area of all WotLD pages and in the image in this page. High density means that exposed survivors will feel more fear when traversing the zone, but armed and skilled survivors will also be able to kill more zombies.

Color codes:

Color Risk Density Notes
Green Low Risk 0-10% Safe to most survivors.
Blue Danger 10-30% Safe to armed survivors. Low risk of injuries.
Yellow Risky 30-50% Moderate risk of injuries.
Orange Deadly 50-70% High risk of injuries.
Red Suicide! 70%-100% Very high of injuries and bites. Avoid if possible.