How many blocks?
Blocks = blocks per course × number of courses
A wall 30 ft long and 3 ft high with 16 × 6 in blocks: 23 blocks per course and 6 courses above ground, plus one buried course, gives 161 blocks and 23 caps.
The first course sits below ground level so the wall cannot slide forward. Most manufacturers ask for at least one buried course, or 1 in of burial for every 8 in of wall height.
Planning the rest of the job? Use the French drain calculator and the Stone calculator too.
Base and drainage
Water building up behind a wall is the main reason walls fail. The free draining gravel and a pipe at the bottom let it escape.
Height limits
Most DIY block systems are designed for walls up to about 3 to 4 ft (1 to 1.2 m) high. Taller walls, or walls holding back a driveway or a slope above, usually need geogrid reinforcement, an engineered design and often a permit.