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This retaining wall calculator estimates the number of segmental wall blocks and cap blocks for a garden or landscape wall, plus the gravel needed for the base trench and the drainage zone behind the wall. Enter the wall size and the dimensions of your block in imperial or metric units.
How to Use the Retaining Wall Calculator
- Enter the wall length and the height you want above ground.
- Enter the face length, height and depth of your block (printed on the product label).
- Keep “bury the first course” ticked; most manufacturers recommend it for stability.
- Tick cap blocks if you want a finished top, and add a waste allowance for cuts.
How the Calculation Works
Courses = Wall height ÷ Block height (rounded up) + 1 buried course
Blocks = Courses × (Wall length ÷ Block length, rounded up) × (1 + waste %)
The base trench is assumed to be 6 in (15 cm) of compacted gravel and 6 in wider than the block on each side. The drainage zone is 12 in (30 cm) of clean gravel behind the full height of the wall. Gravel weight uses about 1.68 tonnes per cubic meter (about 1.4 tons per cubic yard).
Example
A 30 ft wall, 3 ft high, built with 16 × 6 × 12 in blocks needs 6 courses above ground plus 1 buried course, so 7 courses of 23 blocks. With 5% extra that is 170 blocks and 32 cap blocks (12 in long), plus about 1.1 cu yd of base gravel and 3.9 cu yd of drainage gravel.
Common Retaining Wall Block Sizes
| Block (L × H × D) | Blocks per sq ft of wall face | Typical use |
|---|---|---|
| 11.75 × 4 × 6.75 in | about 3.1 | Garden edging, walls under 2 ft |
| 12 × 4 × 8 in | 3 | Small garden walls |
| 16 × 6 × 12 in | 1.5 | Landscape walls up to about 3–4 ft |
| 18 × 8 × 12 in | 1 | Larger walls, faster build |
Building Tips
- Compact the base in 2 in lifts and level the first course carefully; every mistake telescopes upward.
- Set each course back slightly (most blocks have a built-in lip) so the wall leans into the hill.
- Install a perforated drain pipe at the base behind the wall and backfill with clean gravel.
- Glue cap blocks with masonry adhesive.
- Walls over about 4 ft (1.2 m), or walls supporting driveways or buildings, need engineering, geogrid and usually a permit.
Related Calculators
- Gravel calculator for bulk base material.
- Paver calculator for the patio in front of the wall.
- Concrete block calculator for mortared block walls.
Frequently Asked Questions
How many retaining wall blocks do I need?
Divide the wall face area by the face area of one block. For 16 × 6 in blocks that is 1.5 blocks per square foot, plus one buried course and about 5% extra.
How deep should a retaining wall base be?
Most manufacturers recommend a 6 in compacted gravel base, with the first course of blocks buried below ground (about 1 in of burial for every 8 in of wall height, and at least one full course).
How tall can a retaining wall be without reinforcement?
Gravity walls built from standard segmental blocks are usually limited to about 3–4 ft. Taller walls need geogrid reinforcement and an engineer’s design.
Do I need gravel behind a retaining wall?
Yes. About 12 in of clean, angular gravel behind the wall lets water drain and stops pressure from building up and pushing the wall over.
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