Calibre: Create a column with count of words

• Install the plugin "Count Pages" (Preferences / Get plugins)
• Add the column
  • right click a column
  • "Add your own columns"
  • "Add a user-defined" column
    • Lookup name: words
    • Column heading: Words
    • Column type: Integers
    • Format for numbers: {:n}
    • Restart Calibre
• Connect the new column to the new plugin
  • "Count Pages / Customize Plugin"
  • "Word count options: Custom column:" words

If you don't like the formatting of the number with the separator, leave "Format for numbers" just empty. Of course you can use the above to add a column for pages too.