Estimate a segmental block retaining wall from its length, visible height, and block size. The calculator counts courses (including one buried course), blocks with a breakage allowance, cap units and adhesive tubes, the crushed-stone base trench, the drainage gravel behind the wall, and the perforated pipe along its base — and it warns you when the height reaches the range where geogrid, an engineer, or a permit is typically required.
For the gravel itself by area and depth, use the gravel calculator. If the wall frames a patio, the paver calculator handles the surface, and the French drain calculator sizes a drain for water coming down the slope. Ordering the stone by the ton rather than the yard? Our guide to cubic yards, tons, and density explains the conversion and why to ask your supplier what their stone weighs.
Frequently asked questions
How many blocks do I need for a retaining wall?
Divide the wall length by the block length to get blocks per course, rounding up. Divide the visible height by the block height to get courses, round up, and add one course that is buried below grade. Multiply courses by blocks per course and add about 5% for breakage and cuts. A 24 ft long, 2.5 ft tall wall of 16 × 6 in blocks needs 18 blocks per course and 5 visible courses plus 1 buried, so 108 blocks, or 114 with waste. Caps are counted separately along the top.
Why is the first course buried?
Burying the base course keeps the bottom of the wall from kicking out under soil pressure and protects it from erosion. Block manufacturers typically call for burying about 1 inch of block for every foot of wall height, with 6 inches as a minimum — one full course of a 6 or 8 inch block covers that for most garden walls. The calculator adds one buried course by default; uncheck it only if your base is already set and you are counting visible courses alone.
How much gravel goes under and behind a retaining wall?
The base is a trench about twice the block's depth wide (24 inches for a 12 inch deep block) filled with 6 inches of compacted crushed stone. Behind the wall, place about 12 inches of clean drainage gravel for the full height of the blocks so water can drain rather than push on the wall. The calculator estimates both in cubic yards and tons at about 2,800 lb per cubic yard; weights vary by stone, so confirm with your supplier. It does not count gravel used to fill hollow block cores.
When does a retaining wall need an engineer or a permit?
Many building codes require an engineered design and a permit for retaining walls 4 feet or taller, and some measure that height from the bottom of the footing rather than the visible face. Walls of any height need engineering if they hold back a slope, a driveway, a pool, or a building. Unreinforced block walls usually top out around 3–4 feet depending on soil; taller walls need geogrid layers designed for the site. The calculator flags walls at 3 feet and 4 feet, but your local building department's rules decide.
How much adhesive do I need for wall caps?
Cap units are glued to the top course with a landscape-block adhesive. The adhesive maker's data sheet gives coverage as feet of bead per tube: a 10 oz tube of Loctite PL 500 Landscape Block runs about 13.6 ft of 3/8 inch bead, and a 28 oz tube about 38 ft. With two beads along the top course, a 10 oz tube covers roughly 7 ft of wall. The calculator uses those figures by default; change the bead length for your product or bead size.