When using a map, you can measure the area of a polygon, the length of a line, or find the coordinates of a point. The map viewer calculates the shortest distance (using the ellipsoid-based geodesic calculation) even if the data uses a projected coordinate system. Before or after performing each measurement, you can change the default units of measurement. Your administrator may have set the default units for the map, including the measure tool. You can change the units you see by updating your profile.
You may find that what you want to measure is not completely contained within the current extent of the map; for example, a street or parcel may extend beyond what is visible. In order to move the map as you're measuring, while still maintaining the cumulative total distance or area, you can hold down the left mouse button (or the equivalent navigation) and pan the map in the direction you want. Resume measuring by clicking the map with the left mouse button.