Retaining wall calculator

Dig a trench and place 4–6 in of compacted crushed gravel under the first course, and bury the first course about 10% of the wall height (minimum one block). A level, compacted base is 90% of a straight wall.

Yes — water pressure behind the wall is what topples it. Put 12 in of clean crushed gravel behind the blocks, a perforated drain pipe at the base sloped to daylight, and geotextile between gravel and soil.

Most block systems allow 3–4 ft (0.9–1.2 m) unreinforced with a level backfill. Above that, or with a slope or driveway pushing on the wall, you need geogrid reinforcement and an engineered design — local codes often require a permit past 4 ft.

Segmental blocks step back into the slope, typically ¾–1 in per course, via a rear lip or pins. This lean plus the buried first course is what lets a dry-stacked wall resist soil pressure.