ArcGIS Online has the capability to display the shape area and shape length attributes of a feature layer with the December 2016 Update. These are stored in the Shape__Area and Shape__Length fields in the feature layer's attribute table, and update each time a feature is edited or new features are created.
In some instances, the Shape__Area and Shape__Length fields might not visible in the attribute table of the feature layer.
The Shape__Area and Shape__Length fields might not be visible in the attribute table of a feature layer due to the following:
To make the Shape__Area and Shape__Length fields visible, use one of the following options:
Note: This option can also be done from the Data tab of the Item Details of the feature layer.
Note: This option may not work for all hosted feature services and tends to work best when the service was originally published from ArcMap.
Note: When applying edits through Update Layer Definition, the error "Invalid definition for 'LastEditDate'." may be received. To avoid this error, find "lastEditDate" in the syntax and change it to "LastEditDate" prior to clicking Update Layer Definition.
Be aware that the values in the Shape__Area and Shape__Length fields of a feature layer in ArcGIS Online might be different when compared to the values of these fields in the same feature layer in ArcGIS Desktop or ArcGIS Pro. This is because ArcGIS Online uses the WGS 1984 Web Mercator (auxiliary sphere) coordinate system with meters as the units of measurement.
To calculate the area and length of the features with reference to a specific spatial reference in ArcGIS Online, use a custom basemap with the specific projection. Learn more: FAQ: Why are my map, distance and area measurements wrong when using WGS 1984 Web Mercator?