Does this include cap stones?
No, this calculates the main wall blocks only. Cap stones would be calculated based on linear footage.
Remember to bury the first course of blocks for stability. For a 3ft wall, at least 4 inches (approx 1 course) should be buried.
Plan for 12 inches of drainage gravel behind the wall. For this 20ft wall, you'll need approx 2.2 cubic yards of gravel.
You will need approx 14 cap stones to finish the top of the wall.
Blocks = Wall Area / Block AreaWall AreaLength x HeightBlock AreaLength x Height of one block