Each time you zoom in, zoom out, or resize the map, the scale changes. These changes are reported on the scale bar.
The scale bar consists of a representative fraction (RF) (1:24,000), a ruler with distance markers, and an equation (one inch = 40 miles). The fraction (RF scale) is a common way to present scale in cartography and can be read as "one inch on the screen is equal to 24,000 inches on the ground." The equation (sometimes referred to as a "verbal scale") presents the same information but translates the single unit (an inch in this example) into a unit you'd use when getting around in the real world (e.g., miles).
Click the View menu and click Scale Bar.
Setting scale bar properties
Click the View menu and click Scale Bar Properties.
Point to Map Units, Scale Units, and Screen Units to set the appropriate units.
Map units: Map units are the units in which geographic data is stored (data source units). You can set the map units as decimal degrees, feet, or meters.
Scale units: Scale units display in the scale bar itself and in the right side of the verbal scale equation (1 centimeter = 200 kilometers). You can set the scale units as miles, feet, meters, or kilometers.
Screen units: Screen units correspond to the actual display on your computer monitor and are presented on the left side of the verbal scale equation (e.g., 1 inch = 40 miles). You can set the screen units as either inches or centimeters.
Tip: You can also right-click on the scale bar to open the scale bar properties menus.