How many retaining wall blocks?
Count the courses you'll see, then add the buried ones: at least one course, and at least a tenth of the wall's height below ground. Multiply courses by the blocks in each course (wall length ÷ block length) and add about 5%.
Example: a 30-foot wall standing 24" tall with 6" x 18" blocks has 4 exposed courses plus 1 buried — 5 courses of 20 blocks, or 105 blocks with extra. It takes 21 cap units.
The base and drainage are what keep it standing
- Leveling pad: a trench about 12" wider than the block, with at least 6" of compacted 3/4"-minus base.
- Drainage stone: at least 12" of clean 3/4" stone directly behind the blocks and in their cores, the full height of the wall.
- Drain pipe: 4" perforated pipe at the bottom of the stone, run to daylight or a drain.
- Filter fabric: between the stone and the soil so the drainage doesn't clog.
Water pressure behind a wall is the most common reason they lean and fail. For the 30-foot example that's about 1.2 cubic yards of base and 3.1 cubic yards of drainage stone.
Height limits
Most block makers call for layers of geogrid in the backfill above about 3 feet. Walls holding back more than 4 feet of soil — or carrying a driveway, slope or building above — usually need an engineered design and a permit. Check your block maker's chart and local rules.
Building a freestanding garden wall instead? Try the brick and block calculator. Need gravel for a path above it? Use the gravel calculator.
Sources
- SRW Quick Reference Install Guide — Belgard
- Best Practices for SRW — Allan Block