How many retaining wall blocks do I need for a 40-foot wall?

For a 40-foot wall that is 6 courses tall using standard 12-inch-wide blocks: 40 blocks per course x 6 courses = 240 blocks. With a 5% waste factor for a straight wall, order 252 blocks. You will also need 40 cap blocks for the top course.

How much gravel do I need for a retaining wall base?

The gravel base should be 6 inches deep and extend 6 inches beyond the block depth on each side. For a 40-foot wall with 12-inch-deep blocks, the base is 40 ft x 1.5 ft x 0.5 ft = 30 cubic feet = 1.11 cubic yards = approximately 1.56 tons of compacted gravel.

When do I need geogrid for a retaining wall?

Geogrid reinforcement is recommended for retaining walls taller than 3 feet. Geogrid layers are placed every 2 courses and extend behind the wall into the retained soil. The grid length should be at least 75% of the wall height. Without geogrid, taller walls can bow outward under soil pressure.

What waste factor should I use for retaining wall blocks?

Use 5% waste for straight retaining walls where few cuts are needed. For curved walls, use 10% waste because more blocks need to be cut to follow the curve. This calculator lets you adjust the waste factor from 0% to 25% based on your specific project complexity.

Retaining Wall Block Calculator

Estimate blocks, cap blocks, gravel base, drainage aggregate, geogrid, adhesive, and drain pipe for segmental retaining walls. Adjustable waste factors for straight and curved walls.

Wall Configuration

Wall Length (ft)

Courses (rows)

Block Size

Curved wall (increases waste to 10%)

Waste factor5%

5% for straight walls. Increase for curves or complex layouts.

Drainage pipe daylight extension (ft)

Extra pipe length beyond the wall to reach daylight for drainage outflow.

How to Estimate Materials for a Retaining Wall

Why the Gravel Base Matters

Every segmental retaining wall sits on a compacted gravel base, typically 6 inches deep and 6 inches wider than the block depth on each side. This base provides drainage below the first course and prevents frost heave. Skipping or skimping on the base is the number-one cause of retaining wall failure.

Geogrid Reinforcement

Walls taller than 3 feet need geogrid layers to tie the wall into the soil behind it. Grid is placed every 2 courses and extends back into the retained soil at least 75% of the wall height. Without geogrid, taller walls will bow outward under soil pressure within a few seasons.

Drainage Behind the Wall

A 12-inch zone of drainage aggregate behind the wall, combined with a perforated pipe at the base, prevents hydrostatic pressure from building up. Water is the enemy of retaining walls — proper drainage extends wall life from years to decades. Always run the pipe to daylight so water has somewhere to go.

Cap Blocks and Adhesive

Cap blocks give the wall a finished look and protect the top course from water infiltration. They are glued in place with landscape block adhesive — approximately 1 tube per 10 linear feet of wall. Apply adhesive in a zigzag pattern on both the cap and the top course for maximum bond.

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