DIY CalculatorIt is built for dry-stacked segmental retaining wall blocks, the kind sold at home centers for garden walls and raised beds. Change the block size to match the label on the blocks you plan to buy.
Good to know
- Bury the first course: about 1 in for every foot of wall height, and at least one full course.
- Set the first course on a 6 in base of compacted crushed gravel (paver base), about 12 in wider than the block depth.
- Backfill the 12 in behind the wall with clean crushed stone and run a perforated drain pipe at the base — water pressure, not soil, is what pushes most walls over.
- Walls over 4 ft high usually need a permit and an engineered design with geogrid. Check your local code before you build.
- Glue cap blocks down with masonry adhesive.
Wall blocks needed (16 × 6 in face, including one buried course, no extra added)
| Wall length | 2 ft high | 3 ft high | 4 ft high | Cap blocks (16 in) |
|---|---|---|---|---|
| 10 ft | 40 | 56 | 72 | 8 |
| 20 ft | 75 | 105 | 135 | 15 |
| 30 ft | 115 | 161 | 207 | 23 |
| 50 ft | 190 | 266 | 342 | 38 |
| 100 ft | 375 | 525 | 675 | 75 |
Add about 5% extra for cuts and broken blocks. Walls over 4 ft usually need a permit and an engineered design.
How to use this calculator
- Measure the wall length along the front and the height you want above ground.
- Pick your block size (check the face length, height and depth on the label) and whether you want cap blocks.
- The calculator adds buried courses, the gravel base and the drainage stone behind the wall.
- Optionally enter a price per block to estimate the cost.
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Frequently asked questions
How many blocks do I need for a 20 ft long, 3 ft high retaining wall?
With standard 16 in long, 6 in high blocks: 6 courses above ground plus 1 buried course, 15 blocks per course — 105 blocks, or 111 with 5% extra. Add 15 cap blocks at 16 in long.
How much gravel do I need behind a retaining wall?
For that 20 ft × 3 ft wall: about 1 ton of paver base for a 6 in deep leveling pad, and about 3.5 tons of clean crushed stone for a 12 in wide drainage zone behind the blocks.
How many retaining wall blocks per square foot?
Divide 144 by the face area of one block in square inches. A 16 × 6 in face is 96 sq in, so 1.5 blocks per square foot of wall face. Don’t forget the buried course, which isn’t part of the visible face.
How tall can a retaining wall be without reinforcement?
Most segmental block walls can go about 3–4 ft high without geogrid, depending on the block, soil and slope above. Walls over 4 ft usually need an engineered design and a permit.