Free Retaining Wall Blocks calculator with clear step-by-step results.
Block count comes from courses times blocks per course, and the courses figure has to include the base courses buried below grade - typically one for every 200 mm of exposed height. Drainage gravel is sized as a 300 mm zone behind the full wall height, since inadequate drainage is the usual cause of retaining wall failure.
Courses required
Courses = ceil(exposed height / block height) + buried courses
Blocks required
Blocks = ceil(courses x ceil(length / block length) x (1 + waste))
Retaining walls carry significant loads and their failure can be dangerous. Walls above the height permitted locally, or with any surcharge behind them, must be designed by a qualified engineer and permitted.
A common rule is one buried course for every 200 mm of exposed wall, on a compacted granular base at least 150 mm thick. Sloping ground and poor soils need more.
Generally above about 1.2 m exposed height, or any height with a surcharge such as a driveway, slope or structure behind the wall. Most jurisdictions require a permit and design above that threshold.