When using the Calculate Geometry tool to perform calculations on basemaps, inaccurate results are returned.
Online basemaps use the 'Web Mercator' projection, but the 'Mercator' projection is used for navigation. A straight line drawn on the map is the bearing to fly a plane or sail a boat from the starting point to the destination, however this severely distorts distance and area.
The following are possible solutions to the issue.
Assign a more suitable projection to the ArcMap data frame
Refer to the following article, Select a suitable map projection or coordinate system, for more information. The article is linked to the Projections Table, which lists all supported map projections in ArcGIS for Desktop and their properties.
Apply the selected projection to the ArcMap data frame
Get the accurate calculation output