I have a shapefile with lots of line routes.
I projected it and calculated the length using calculate geometry.
How can I get the sum of all these routes in ArcGIS Desktop?
The easiest way is to right click on the field header (e.g. "line" in this example) and choose "Statistics...".
The statistics window below will pop up with your information.
You can also use the "Summarize" command in the same menu for more advanced summary statistics.
If you have the features selected when you right click on the field that you want to see the total sum of choose Statistics and the sum will be displayed for the features that are selected.