Retaining Wall Block Calculator: blocks, caps, base gravel and drainage stone
Count the segmental retaining-wall blocks and cap blocks for a wall from its length, height and block size, with a buried course, waste, base gravel, drainage stone, drain pipe, geogrid estimate, pallets and material cost.
How it works
Blocks per course = ⌈wall length ÷ block face length⌉ · courses = ⌈(wall height − cap height if the cap is inside the height) ÷ block height⌉ (+ buried courses = max(1, ⌈10% of height ÷ block height⌉) if added) · blocks = per course × courses × (1 + waste %) · caps = ⌈length ÷ cap length⌉ · base gravel = length × trench width × base depth × 1.2 · drainage stone = length × stone width × wall height × 1.1
A segmental retaining wall is a grid: the blocks in one course cover the length, and the courses cover the height, so the block count is the two multiplied together, each rounded up because a part block still has to be bought. Waste covers cut blocks at the ends, corners and curves and the odd chipped unit. Cap blocks finish the top; choose whether they sit on top of the height you entered or form the top row inside it (as some calculators assume). The bottom course goes below grade, about 1 in per foot or 10% of the wall height and at least one full block, on a trench of compacted crushed gravel at least 24 in wide (or 12 in wider than the block) and 6 in deep. Behind the wall, a 12 in column of clean crushed stone with a perforated drain pipe at the bottom and filter fabric over it relieves water pressure, which is the most common reason small walls lean or fail. Gravel is ordered loose, so the base is increased 20% for compaction and the drainage stone 10%; tonnage uses about 1.4 tons per cubic yard of crushed stone. Over about 4 ft (or with a slope, driveway or building above), walls need geogrid reinforcement and usually an engineer’s design and permit; the geogrid line is only a rough estimate based on layers no more than 16 in apart reaching back 60% of the height.
Worked example
A 20 ft long, 3 ft high wall with 12 × 8 in blocks, 5% waste, caps on top: 240 in ÷ 12 in = 20 blocks per course, and 36 in ÷ 8 in = 4.5, rounded up to 5 courses. That is 20 × 5 = 100 blocks, and 100 × 1.05 = 105 with waste, plus 240 ÷ 12 = 20 caps; the finished wall is 40 + 4 = 44 in tall including the cap. A 24 in wide, 6 in deep base is 240 × 24 × 6 ÷ 46,656 = 0.74 cu yd compacted, so order 0.74 × 1.2 = 0.89 cu yd (about 1.24 tons). A 12 in drainage column 40 in tall is 240 × 12 × 40 ÷ 46,656 = 2.47 cu yd, or 2.72 cu yd (about 3.80 tons) to order, with 20 ft of drain pipe.
Sources: Allan Block: typical retaining wall construction (base, burial, wall rock, geogrid) · Inch Calculator: retaining wall calculator · Omni Calculator: retaining wall calculator
Frequently asked questions
How many retaining wall blocks do I need?
Divide the wall length by the block face length to get blocks per course, divide the height by the block height to get the number of courses, round both up and multiply. A 20 ft by 3 ft wall of 12 × 8 in blocks takes 20 × 5 = 100 blocks, about 105 with 5% waste.
How deep should the first course be buried?
A common rule is 1 in of burial per foot of wall height, or about 10% of the height, and never less than one full block or about 6 in. Pick “Add below grade” if the height you entered is only the exposed part.
How much gravel goes under a retaining wall?
Block makers call for a trench at least 24 in wide (or 12 in wider than the block depth) with 6 in of compacted crushed gravel. For a 20 ft wall that is about 0.74 cu yd compacted, or roughly 0.89 cu yd ordered loose.
Do I need drainage stone and a drain pipe?
Yes for almost every retaining wall. Water pressure behind the wall is the main cause of bulging and tipping. Place about 12 in of clean crushed stone directly behind the blocks for the full height, with a 4 in perforated pipe at the base that drains to daylight and filter fabric between the stone and the soil.
Do I need cap blocks?
Caps give the wall a finished top edge and are glued down with concrete adhesive. They are optional structurally but standard on decorative walls. Count one cap per cap length along the wall; caps on curves may need cutting.
When does a retaining wall need geogrid or an engineer?
Many block makers and local codes set the limit at about 3 to 4 ft of total height, less with a slope, driveway, fence or building above the wall. Taller walls need geogrid layers and usually an engineered design and permit.
Related calculators
Estimates only: add for waste and confirm against local building codes and the block maker’s installation guide. Tall or loaded walls need an engineer.