Retaining wall calculator
A block retaining wall is mostly what you cannot see: a compacted gravel footing, a buried first course, and a column of drainage stone behind the blocks. Enter the wall size and block style and the calculator counts the blocks and caps, then sizes the base and backfill so the wall stays put.
How the retaining wall calculator works
The calculator adds a buried course to the visible height, because the first row of any block wall sits below grade by at least one block height or 1 inch per 8 inches of wall, whichever is more. It divides total height by the block height to get courses and wall length by block width to get blocks per course, then adds 5 percent for cuts at corners and ends.
Below the wall it figures a compacted gravel footing one foot wider than the block, and behind the wall a 12 inch column of clean drainage stone running the full height. Those two volumes, plus perforated pipe and fabric, are what separate a wall that lasts 30 years from one that leans in three.
Formula
courses = ceil((height above grade + buried depth) ÷ block height)
blocks = ceil(wall length × 12 ÷ block width) × courses × 1.05
base gravel cu yd = length × (block depth + 12 in) ÷ 12 × base depth ÷ 12 ÷ 27 × 1.1
Example: a 30 ft wall standing 24 in tall with 12 × 4 in blocks buries one 4 in course, so it is 7 courses of 30 blocks, or 221 blocks with extra, plus 32 caps. The 6 in base takes about 1.0 cubic yard of gravel and the 12 in drainage column behind the wall another 2.9 cubic yards of clean stone.
Building it so it lasts
- Dig the trench to base depth plus the buried course, one foot wider than the block. Compact the soil.
- Base: spread 3/4 in minus crushed gravel in 2 to 3 in lifts and compact each with a plate compactor. Level it perfectly; every course above copies the first.
- First course: set each block, check level front to back and side to side, and tap to adjust. Take your time here.
- Drainage: lay perforated pipe behind the first course sloped to an outlet, wrap the backfill zone in fabric, and fill behind each course with clean 3/4 in stone, never soil.
- Stack with joints staggered, pins or lips engaged, and sweep each course clean before the next. Glue the caps with construction adhesive.
Price the gravel by the ton with the gravel calculator. If a poured wall makes more sense, the concrete calculator handles the footing.
Common sizes, already worked out
Common questions
How many retaining wall blocks do I need for a 20 ft wall?
For a wall 2 ft tall with standard 12 × 4 in blocks, about 150 blocks and 21 caps. That is 7 courses including one buried course, with 20 blocks per course plus 5 percent for cuts.
How deep should the base be for a retaining wall?
At least 6 in of compacted gravel for most walls, plus a buried first course. Use 4 in for garden walls under 2 ft and 8 in or more for tall walls or soft, wet soil.
Do I need drainage behind a retaining wall?
Yes, on anything taller than a foot or two. Water trapped behind a wall is the main reason walls fail. Use 12 in of clean crushed stone behind the blocks and a perforated pipe at the base that drains to daylight.
How tall can I build a retaining wall without an engineer?
Most jurisdictions allow up to 3 or 4 ft without an engineered design or permit, but check locally. Walls holding back a slope, a driveway, or another wall above them often need engineering even when shorter.