How to use the retaining wall calculator
- Measure the wall's total length along its face in feet. For a curved wall, run a garden hose along the line and measure the hose.
- Enter the height of the wall above the finished ground. The calculator adds one buried course below grade.
- Choose the block's face size: the width and height you see on the front of the wall. It's printed on the label or product page.
- Enter the cap length, or 0 if you're leaving the top course uncapped.
Retaining wall formulas
blocks per course = wall length (in) ÷ block width (in)
courses = wall height (in) ÷ block height (in) + 1 buried course
blocks = blocks per course × courses × 1.05
caps = wall length (in) ÷ cap length (in)
The 5% extra covers broken blocks and cuts at the ends. For base gravel, the calculator assumes a trench 24 inches wide filled with 6 inches of compacted crushed stone. For drainage, it assumes a 12 inch wide zone of clean gravel behind the wall for its full height. Both include 15% extra for compaction.
Blocks for common wall sizes
Includes one buried course and 5% extra. Gravel is base plus drainage for 16 × 6 in blocks.
| Wall | 12 × 4 in blocks | 16 × 6 in blocks | Caps (12 in) | Gravel (tons) |
|---|---|---|---|---|
| 10 ft long × 2 ft high | 74 | 42 | 10 | 2.1 |
| 20 ft long × 2 ft high | 147 | 79 | 20 | 4.2 |
| 20 ft long × 3 ft high | 210 | 111 | 20 | 5.4 |
| 30 ft long × 2 ft high | 221 | 121 | 30 | 6.3 |
| 30 ft long × 3 ft high | 315 | 170 | 30 | 8.0 |
| 50 ft long × 3 ft high | 525 | 280 | 50 | 13.4 |
Building a block wall that lasts
- Base: dig a trench about twice the block depth wide, lay 6 inches of compacted crushed stone, and level it carefully. The first course decides how straight the whole wall is.
- Bury the first course. A common rule is to bury at least one course, or about 1 inch for every 8 inches of wall height, whichever is more.
- Drainage: backfill the 12 inches behind the wall with clean, angular gravel and lay a perforated drain pipe at the bottom that exits at the ends. Water pressure is what pushes most walls over.
- Stagger the joints so each block sits over the seam between two blocks below.
- Glue the caps on with masonry adhesive.
Walls taller than about 3 to 4 feet, or walls holding up a driveway, a slope or a building, usually need geogrid reinforcement, an engineer's design and a permit. Check your local code before you build.
Frequently asked questions
How many retaining wall blocks do I need?
Divide the wall length in inches by the block width to get blocks per course, then multiply by the number of courses (height ÷ block height, plus one buried course). Add 5% for breakage.
How many retaining wall blocks are in a square foot?
For 16 × 6 in blocks it is 1.5 blocks per square foot of wall face. For 12 × 4 in blocks it is 3 per square foot, and for 18 × 6 in blocks it is 1.33.
How much gravel do I need behind a retaining wall?
Plan on a 12 inch wide zone of clean gravel behind the wall for its full height. For a 20 ft long wall standing 3 ft high (3.5 ft counting the buried course) that is about 3 cubic yards, or roughly 4.2 tons with compaction.
How tall can a retaining wall be without reinforcement?
Most segmental block walls can go about 3 to 4 feet high without geogrid, depending on the block and the soil. Many towns require a permit and an engineer above 4 feet.