Retaining Wall Materials: How Many Blocks and How Much Gravel

A segmental retaining wall (the kind built from interlocking concrete blocks) looks simple, but the blocks are only part of the materials. A good wall also needs a compacted stone base, a drainage zone of clean stone behind it, and a drain pipe. This guide shows how to estimate all of it, and where a "weekend project" becomes one that needs an engineer.

Counting the blocks

1. Courses (rows). Divide the wall height by the block height, rounding up. A 3 ft wall of 4 in blocks is 36 ÷ 4 = 9 courses. Include the buried first course in the height.
2. Blocks per course. Divide the wall length by the block length, rounding up. 20 ft of 12 in blocks is 20 per course.
3. Total. Courses × blocks per course, plus 5% for breakage and cuts: 9 × 20 = 180, so 189 blocks.
4. Caps. One cap block for each block-length along the top, in addition to the wall blocks: 20 more here.

The retaining wall calculator does the block, gravel and pipe counts for any size of block.

Quantities for common walls (12 × 4 in blocks, 10 in deep)

Larger blocks mean far fewer pieces to place: an 8 in tall, 16 in long block covers 128 in² of face, which is 2.7 times the area of a 12 × 4 in block. Use the calculator with your own block dimensions.

The gravel: base and drainage

Two separate stone layers go in:

• Base. A compacted bed of crushed stone (road base or similar) under the first course, typically at least 6 inches deep and a few inches wider than the blocks, so every block sits level and firm. Allow about 20% extra for compaction.
• Drainage zone. Behind the wall, at least 12 inches of clean, angular stone (3/4 inch) for the full height of the wall. Water pressure is the main thing that pushes a wall over, so this layer lets water drain freely to the pipe.

At the bottom of the drainage stone, behind the blocks, lay perforated drain pipe with the holes down, sloping gently to an outlet (a daylight exit or a dry well). Wrap the stone in filter fabric so that soil does not wash in and clog it. The calculator counts the pipe as the wall length, and the stone as 12 in wide for the whole wall.

Building it right

• Excavate for the base trench and the drainage zone before you start.
• Compact the base and check that it is level in both directions. Every course follows the first one.
• Bury the first course (or at least 6 inches), so the wall is anchored into the ground.
• Offset joints like brickwork, and follow the block maker's setback (the lean into the slope) and any pins or adhesive.
• Backfill and compact in lifts of about 6 to 8 inches as you go, with the drainage stone against the wall and soil behind it.
• Finish with cap blocks, glued with block adhesive.

When you need an engineer or a permit

A low garden wall is a reasonable DIY project. But there is a point at which the forces become serious: a cubic yard of wet soil weighs well over a ton. Many places require a permit, and an engineered design with reinforcing (geogrid), for:

• Walls over about 3 to 4 feet in height.
• Walls that support a driveway, building, pool or slope above them.
• Terraced walls, or walls close to a property line or a structure.
• Any wall where failure would endanger people or neighbors.

Block manufacturers publish installation guides and limits for their products. Treat the numbers on this page as a materials estimate, not a design.