To use this feature, first go to the File menu, and select Spreadsheet settings. Check that your spreadsheet's Locale setting is United States. Then, highlight the cells of data that you'd like to total (up to 1000 cells).
The sum will show in the bottom right corner of the page:
Note: The quick sum feature is enabled based on your location and may not work for certain number or currency formats. If you chose a non-English language in the Spreadsheets language settings, you can still use the quick sum feature when you're working with a spreadsheet that's in English.