Construction

Retaining Wall Block Calculator

The retaining wall block calculator tells you how many wall blocks, cap blocks and cubic yards of gravel you need for a segmental retaining wall. Enter the wall length and height and the face size of your block, add a waste allowance, and get a shopping list.

Blocks & caps
Base & drainage gravel
Feet or metres
Updated Nov 5, 2026

How the quantities are worked out

The wall is divided into rows (courses) and blocks per row. Courses = wall height ÷ block height, rounded up, and one course is usually the cap. Blocks per course = wall length ÷ block face length, rounded up. A buried base course goes below grade for stability, and walls over about 4 feet normally need an engineer and geogrid reinforcement.

Item	How it is calculated
Wall blocks	Courses × blocks per course, plus waste
Cap blocks	Wall length ÷ cap length, plus waste
Base gravel	Length × (block depth + 12 in) × base depth
Drainage gravel	Length × wall height × drainage depth behind the wall

Retaining walls hold back soil and water. Walls taller than about 4 feet, walls supporting a slope, driveway or building, or walls in poor soil need engineered design and often a permit.

Example. A 30 ft long, 3 ft high wall with 16 in × 6 in blocks and a 6 in buried course: 7 courses (including the buried one) of 23 blocks, minus one course for the cap, is 6 × 23 = 138 blocks plus 23 caps. With a 10% waste allowance, order 152 blocks and 26 caps.

Questions people ask

How much extra should I order?

Ten to fifteen percent covers cuts, breakage and corners. Curved walls need more.

Do I need drainage gravel?

Yes. A layer of clean crushed stone behind the wall, with a perforated pipe at the base, stops water pressure from pushing the wall over.

How deep is the base?

Typically 6 inches of compacted gravel, wider than the block by a few inches each side, with the first course buried.