you are using projected data I hope.
from the help
How Kernel Density works—Help | ArcGIS Desktop
Now if you want to use a multiplier, there are options
You can control the density units by manually selecting the appropriate factor. To set the density to be in meters per square meter (instead of the default of kilometers per square kilometer), set the area units to Square meters. Similarly, to have the density units of your output in miles per square mile, set the area units to Square miles.
