Retaining Wall Block Count: How Many Blocks, Gravel, and Caps You Need

Step 1: Courses and Blocks per Course

Courses = wall height ÷ block height. Blocks per course = wall length ÷ block length. Multiply to get the total. For stepped walls or curves, add by sections. Remember that the first course is buried partly below grade to anchor the wall. A common rule is to bury at least 6 inches of the wall, or 1 inch for each foot of wall height, plus the base.

If your wall is designed to be 3 feet of visible height, the buried course adds one more: 10 courses instead of 9, or 300 blocks plus waste.

Step 2: The Gravel Base

The base is typically 6 inches of compacted crushed stone, extending at least 6 inches in front and the full block depth behind. For a 30-foot wall with 12-inch-deep blocks, a base 24 inches wide and 6 inches deep needs 30 × 2 × 0.5 = 30 cubic feet, about 1.1 cubic yards (roughly 1.6 tons). Compact it in 3-inch lifts and make sure it is level, because every course above follows the base.

A level base is the single most important step, so spend time on it.

Step 3: Drainage Stone and Pipe

Water pressure behind a wall is the main cause of failure. Fill a column at least 12 inches wide behind the wall with clean, angular drainage stone (no fines), up to within 6 to 12 inches of the top. For the example, that is 30 × 1 × 3 = 90 cubic feet, 3.3 cubic yards. A perforated drain pipe at the base, sloped to daylight, carries the water away. Place landscape fabric between the stone and the native soil so that soil does not clog it.

Cap the drainage with 6 to 8 inches of compacted soil or clay to keep surface water from soaking in.

Step 4: Caps, Adhesive, and Geogrid

Cap blocks finish the top and are glued with masonry adhesive. Estimate one per foot for standard 12-inch caps. For walls above about 3 to 4 feet, many manufacturers call for geogrid, a reinforcing mesh placed between courses and extending back into the soil at specified distances, commonly 60% to 100% of the wall height.

Walls taller than about 4 feet, walls that support driveways or structures, and walls in poor soil should be engineered. Many jurisdictions require a permit and engineered plans above a certain height, often 4 feet.

Setback and Batter

Segmental blocks are designed to step back slightly with each course, creating a lean toward the hill called batter, typically about ½ to 1 inch per course. This stabilizes the wall. Do not stack them vertically. Check the manufacturer's installation guide for exact requirements.

Project Checklist

Call before you dig to locate utilities, compute materials with a 5% to 10% allowance, rent a plate compactor, and plan delivery of heavy materials. Backfill in layers as you build, compacting each lift. A slow, careful build lasts decades.