Your wall
Base and drainage assumptions
Typical gravity-wall guidance: a compacted base about 6 in deep and 6 in wider than the block on each side, and clean crushed stone about 12 in deep behind the blocks. Follow your block maker's install guide where it differs.
Wall blocks you need
–
- Cap blocks–
- Finished height–
- Base gravel–
- Drainage stone–
- Perforated drain pipe–
- Base trench–
Estimate only. A block count is not a structural design.
This wall is over 4 ft. Walls this tall usually need a permit and a design from a licensed engineer, often with geogrid reinforcement. Don't build it from a block count.
Walls over about 4 ft, or holding back a slope, driveway or building, usually need a permit and an engineer's design. Check with your local building department. Call 811 before you dig.
Order it
We earn a commission if you buy through these links, at no extra cost to you. As an Amazon Associate we earn from qualifying purchases.
How the retaining wall calculator works
Blocks: blocks per course = wall length ÷ block length, rounded up. Courses = (height above ground − cap height) ÷ block height, rounded up, plus one buried course if you tick that box. Blocks = courses × blocks per course, plus your extra for cuts. A 20 ft wall, 2 ft high, with 12 × 4 in blocks and a 4 in cap is 5 courses of 20 blocks above ground, 100 blocks, plus 20 more for the buried course and 20 caps.
Base gravel: a trench as long as the wall, as wide as the block plus 12 in, and 6 in deep, filled with compacted crusher run or paver base.
Drainage stone: a zone of clean crushed stone (such as #57) behind the blocks, as deep as you set (12 in by default) and as tall as the block courses. A perforated drain pipe runs along the back at the bottom, so the pipe length is the wall length. Water behind a wall is the usual reason small walls lean or fail.
Block sizes vary by maker and many are nominal (a "12 in" block may be 11.75 in), so enter the sizes on the pallet or product page you're buying from. Corners, curves and steps change the count; order a few spare.
How many retaining wall blocks do I need?
Blocks per row = wall length ÷ block length, rounded up. Rows = wall height (less the cap) ÷ block height, rounded up, plus one buried row. Multiply the two and add about 5% for cuts and breakage.
How much gravel do I need behind a retaining wall?
A common rule is clean crushed stone about 12 inches deep behind the blocks, running the full height of the wall. For a 20 ft wall 2 ft high that's about 40 cu ft, or 1.5 cubic yards.
How tall can a retaining wall be without an engineer?
It depends on where you live. Many US codes need a permit and an engineered design for walls over about 4 ft, or for shorter walls holding back a slope, driveway or building. Check with your local building department before you build.
Should the first row of retaining wall blocks be buried?
Yes. Block makers' guides generally bury the first course below grade, with more burial for taller walls, so the wall doesn't slide or wash out at the base.