How to use the retaining wall calculator
- Measure the wall length along the face, in feet. For curved walls, lay a hose along the line and measure the hose.
- Enter the exposed height above the finished ground.
- Keep the buried row. The first course sits below grade in the trench for stability; manufacturers usually require one buried row per foot or so of wall height on taller walls.
- Choose the block face size. It is the visible width and height of one block, listed on the product page.
Retaining wall formulas
Rows = wall height (in) ÷ block height + buried rows
Blocks = blocks per row × rows × (1 + waste %)
Base gravel = length × 2 ft wide × 0.5 ft deep ÷ 27
Drainage stone = length × 1 ft × total wall height ÷ 27
Worked example
A 30 ft long, 3 ft tall garden wall built with 12 × 4 inch blocks needs 30 blocks per row and 9 visible rows plus 1 buried row, for 300 blocks, or 315 with 5% extra. You also need 30 cap blocks. The 6 inch deep, 24 inch wide gravel base is 1.1 cubic yards, and a 12 inch drainage zone behind the visible wall takes about 3.3 cubic yards of clean stone, or a little more to include the buried row.
Building a wall that lasts
- Base first. Dig a trench about 24 inches wide, add 6 inches of compacted crushed stone, and level the first course perfectly. Most wall failures start at the base.
- Drainage behind the wall. Use clean angular stone behind the blocks and a perforated drain pipe at the bottom so water cannot build up pressure.
- Setback. Most blocks have a lip that steps each row back slightly into the hill. Do not remove it.
- Height limits. Gravity walls with small blocks are usually limited to about 3 to 4 ft. Taller walls need geogrid reinforcement, an engineer and often a permit.
- Call 811 before digging the trench.
Need more base material or topsoil for the planting bed behind the wall? Use the gravel and topsoil calculators.
Estimates for planning only. Follow the block manufacturer's installation guide and local building code.
Frequently asked questions
How many retaining wall blocks do I need?
Divide the wall length by the block width to get blocks per row, divide the height by the block height to get rows, add the buried row, and multiply. A 30 ft long, 3 ft tall wall of 12 × 4 inch blocks needs about 300 blocks plus 5% extra.
How much gravel do I need for a retaining wall base?
A trench about 24 inches wide with 6 inches of compacted gravel uses roughly 1 cubic yard per 27 feet of wall.
How tall can a retaining wall be without an engineer?
Many codes allow gravity block walls up to about 3 to 4 feet without engineering. Anything taller, or walls supporting driveways or slopes, usually needs an engineered design and a permit.
How deep should the first row of retaining wall blocks be buried?
Usually at least one full block, or about 10% of the wall height, whichever is more. Check the manufacturer's instructions.
Do retaining walls need drainage?
Yes. Clean stone backfill at least 12 inches thick and a perforated drain pipe at the base relieve water pressure, which is the main reason walls bulge or fail.
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Last updated October 2026.