i use a quick approximation for calculating mulch: 1
yard covers 100 square feet 3" deep. the math works out to 108 square feet, but for simple calculations in the field, it's close
enough. so yeah, if your area is 2100 square feet, and your depth is 6 inches...in nice round numbers, 40 yards.
there is an
art to math of calculating mulch in volume to spread in area. there used to be a couple cartoons we kept in the office.