Retaining Wall Calculator

How to Use the Retaining Wall Calculator

Building a retaining wall requires careful material planning to avoid costly trips back to the supply yard. Enter the total wall length and desired height in feet, along with the block dimensions in inches. The calculator determines the number of courses (rows), blocks per row, and total blocks needed including a 10% waste allowance for cuts and breakage. It also calculates the gravel base and backfill material in cubic yards, which is how landscape suppliers sell bulk materials.

Standard retaining wall blocks are 6 inches high and 16 inches long, but many manufacturers offer different sizes. The most common blocks weigh 25 to 50 pounds each and feature a built-in lip or pin system that automatically sets the wall batter (the slight backward lean that provides structural stability). Cap blocks are placed along the top row and are typically glued in place with landscape adhesive.

Building a Proper Retaining Wall Foundation

The gravel base is the most critical element of a retaining wall. Dig a trench 6 inches deep and approximately 24 inches wide, which is wider than the blocks to provide stability. Fill with compactable gravel (typically 3/4-inch minus or road base) and compact it in lifts using a plate compactor or hand tamper. The base must be perfectly level along its entire length. An uneven base will cause every subsequent course to be off, resulting in visible gaps and structural weakness.

Backfill and Drainage Tips

Proper drainage behind the wall prevents hydrostatic pressure, which is the number one cause of retaining wall failure. Place perforated drain pipe (4-inch corrugated or rigid PVC) at the base of the wall, behind the first course, with the perforations facing down. Cover the pipe with clean 3/4-inch gravel and continue the gravel backfill to within 6 inches of the top. Use landscape fabric between the gravel and native soil to prevent fine particles from clogging the drainage layer. The top 6 inches can be filled with topsoil for planting.

Construction Best Practices

Stagger the joints between courses, just like bricks in a wall, so that each block spans the joint of the course below. This interlock provides structural strength. Use landscape adhesive on the cap blocks and on any course where the built-in lip system is cut away. For walls over two feet tall, install geogrid reinforcement every two to three courses. The geogrid extends into the soil behind the wall and anchors it against the soil pressure. Compact the backfill every 6 to 8 inches as you build up to prevent settling. Check for level and alignment at every course before proceeding to the next.

Frequently Asked Questions

How many blocks do I need for a retaining wall?

Divide the wall height in inches by the block height for the number of rows. Divide the wall length in inches by the block length for blocks per row. Multiply rows by blocks per row and add 10% for waste. Standard blocks are 6 inches high by 16 inches long.

How much gravel do I need for the base?

The gravel base should be 6 inches deep and 24 inches wide. Multiply 0.5 feet by 2 feet by the wall length in feet, then divide by 27 to convert to cubic yards. This compacted gravel provides drainage and a level foundation.

How much backfill do I need?

Backfill is typically 12 inches deep behind the wall for its full height. Multiply the wall height by 1 foot by the wall length, then divide by 27 for cubic yards. Use clean gravel for the bottom two-thirds and topsoil for the upper third.

Do I need a permit for a retaining wall?

Most jurisdictions require a building permit for retaining walls over 4 feet high. Walls near property lines or supporting driveways and structures may require engineering regardless of height. Always check local building codes before starting.

How high can I build without engineering?

Most residential retaining walls up to 3-4 feet can be built without professional engineering. Walls over 4 feet typically require engineered plans, a permit, and inspection. Gravity walls without reinforcement should generally not exceed 3 feet. Taller walls need geogrid reinforcement and proper drainage.