Blocks by the face area
Segmental retaining wall blocks show a 16-inch face and stand about 6–9 inches high depending on the system; the calculator counts courses by height and blocks per course by length. Every course buried (the first is set below grade) is still a block you buy — add buried courses to the height.
Backfill is half the material bill
A retaining wall is a drainage system that happens to hold soil: <strong>1 foot of clean gravel behind the entire wall</strong>, with a perforated drain pipe at the base, is what keeps hydrostatic pressure from pushing it over. The calculator returns that gravel volume — order it along with the blocks, and cap blocks per linear foot.