Retaining wall calculator

Divide the wall length in inches by the block's face width to get blocks per course, and divide the wall height in inches by the block height to get courses. Add the buried course, multiply courses by blocks per course, and add about 5 percent for cuts and breakage. A 30 foot wall, 3 feet tall, with 16 inch wide by 6 inch tall blocks is 6 exposed courses plus 1 buried, 7 courses of 23 blocks, or 161 blocks before extra.

Many block manufacturers call for burying about 10 percent of the wall height, with a minimum of around 6 inches, which is often one full course. Their installation guide sets the real number for that block, so follow it.

Block wall guides commonly show a compacted crushed stone leveling pad around 6 inches deep and about 24 inches wide, or twice the block depth. Multiply wall length by pad width and depth in feet, then divide by 27 for cubic yards.

Most block walls call for clean drainage aggregate behind the blocks, often about 12 inches wide for the full height, plus a perforated drain pipe at the base so water does not build up pressure behind the wall. Water behind a wall is a leading reason walls lean and fail.

Rules vary by town and state. Many places require an engineered design and a permit once a wall passes about 4 feet of height, or when it holds up a driveway, slope or building. Taller walls usually also need geogrid reinforcement. Check local code before building.