Retaining Wall Calculator
Wall blocks, caps, base gravel, drainage stone, drain pipe and filter fabric for a dry-stacked segmental block wall, including the buried course most quick calculators forget.
Blocks = (wall length in inches ÷ block length) × (courses above ground + buried courses). Bury at least one course. Under the wall, lay 6 in of compacted gravel; behind it, 12 in of clean drainage stone the full height. A 10 ft long, 2 ft high wall of 6 × 16 in blocks needs about 40 blocks and 1.3 cu yd of stone.
Blocks and stone per 10 ft of wall
Includes one or more buried courses and the gravel base and drainage zone described above. Multiply by your length in feet ÷ 10, and add 5% for blocks.
| Block | 1 ft exposed | 2 ft exposed | 3 ft exposed |
|---|---|---|---|
| Small garden wall block, 4 × 12 × 7 in | 40 blocks · 0.79 cu yd stone | 70 blocks · 1.16 cu yd stone | 100 blocks · 1.53 cu yd stone |
| Standard wall block, 6 × 16 × 12 in | 24 blocks · 0.93 cu yd stone | 40 blocks · 1.30 cu yd stone | 56 blocks · 1.67 cu yd stone |
| Large wall block, 8 × 18 × 12 in | 21 blocks · 1.11 cu yd stone | 28 blocks · 1.36 cu yd stone | 42 blocks · 1.85 cu yd stone |
How to build it so it lasts
- Dig the trench about 12 in wider than the block depth and deep enough for 6 in of base plus the buried course.
- Compact the base in 2–3 in lifts with a plate compactor and level it carefully. The first course decides how straight the whole wall is.
- Lay the first course level both ways, then backfill in front of it.
- Stack and drain as you go: lay each course with the joints staggered, fill the cores and the 12 in zone behind with drainage stone, and compact the soil behind that every course or two.
- Cap and finish: glue caps with landscape block adhesive and top the drainage stone with fabric and a few inches of soil so surface water runs off.
Building a mortared wall instead? Use the concrete block calculator.
Frequently asked questions
How many retaining wall blocks do I need?
Divide the wall length in inches by the block's face length to get blocks per course, then multiply by the number of courses: the exposed height divided by the block height, plus at least one buried course. For a 10 ft wall, 2 ft high, with 6 × 16 in blocks that is 8 blocks × 5 courses = 40 blocks, plus 5% for cuts.
How deep should the gravel base be for a retaining wall?
Most block makers call for at least 6 in of compacted crushed gravel in a trench about 12 in wider than the block is deep, with the first course buried. Bury at least one full course, or one-tenth of the exposed height if that is more.
How much gravel goes behind a retaining wall?
Fill a zone about 12 in wide behind the blocks with clean ¾ in drainage stone for the full height of the wall. For each 10 ft of a 2 ft high wall built with 6 × 16 in blocks, base and drainage stone together come to about 1.3 cu yd (about 1.8 tons).
How tall can a block retaining wall be without reinforcement?
It depends on the block, soil and what sits above the wall, so check the manufacturer's design chart. Many segmental blocks need geogrid above about 3 ft. The International Residential Code (R404.4) requires a designed wall when it retains more than 4 ft of unbalanced soil, and many towns also require a permit at that height.
Do I need a drain pipe behind a retaining wall?
Water pressure is the main reason block walls fail. A 4 in perforated pipe at the base of the drainage stone, sloped to daylight at one end, is recommended for walls over about 2 ft, in clay soil, or where downspouts or slopes send water toward the wall.
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Base, buried-course and drainage-zone dimensions follow common segmental retaining wall practice described by the Concrete Masonry and Hardscapes Association and block makers' installation guides; your block's own guide takes priority. Stone is converted at 1.4 tons per cubic yard; filter fabric adds 1 ft per wall for overlap. Block sizes and weights are typical of each class; enter your exact block under "Custom size". The height limit cites International Residential Code section R404.4. Results are estimates for planning and ordering, not an engineered design.