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Retaining Wall Calculator

Use this retaining wall calculator to estimate how many segmental blocks, cap blocks and how much gravel a wall needs. Enter the wall length and height, then set the block size, waste and an optional cap length. It gives the courses, blocks per course, blocks to buy, wall face area, the gravel base and the drainage gravel behind the wall in cubic yards and tons, and an optional cost. Block size and gravel depths are typical starting values you can edit. For bagged gravel use the gravel calculator, for footings and posts the concrete calculator, and for the wall area the square footage calculator.

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Block size (typical starting values, editable)

Typical, varies by material and moisture. Adjust to match your supplier.

Wall face area
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Courses
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Blocks per course
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Blocks before waste
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Blocks to buy (with waste)
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Cap blocks (with waste)
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Recommended burial depth
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Buried courses added
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Gravel base
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Drainage gravel behind wall
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Total gravel
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Gravel weight (US tons)
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Courses = height ÷ block height and blocks per course = length ÷ block face length, both rounded up. Blocks = courses × blocks per course × (1 + waste), rounded up. If you enter the exposed height, the buried courses are added: at least 6 in, or 1 in per ft of wall height if that is more (Allan Block), rounded up to whole courses. Gravel base = length × trench width × depth, and drainage gravel = length × height × depth behind the wall. The Versa-Lok Standard block (16 × 6 × 12 in), a 6 in base, a trench 12 in wider than the block depth and 12 in of gravel behind the wall (Allan Block) are typical starting values, not standards, so edit them to match your block and plan. Gravel weight uses a typical 1.4 US tons per cu yd. Block cores, compaction, drain pipe, geogrid and adhesive are not counted. This tool counts materials, not stability: walls over about 4 ft, or walls holding up a slope, driveway or building, often need a permit and an engineer, so check local code.

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How it works

How many retaining wall blocks do I need?
Divide the wall length by the block face length and round up for blocks per course. Divide the height by the block height and round up for courses. Multiply the two and add waste. A wall 20 ft long and 3 ft high built with 16 in × 6 in blocks is 15 blocks per course × 6 courses = 90 blocks, or 99 with the default 10% extra. The 10% is an adjustable allowance for cuts and breakage, not a standard value.
How deep should the first course be buried?
Allan Block calls for buried block of at least 6 in, or 1 in for each 1 ft of wall height if that is more. A 3 ft wall buries 6 in, which is one course of a 6 in block, and an 8 ft wall buries 8 in. Home Depot Canada's guide instead buries half of the first course for walls up to 24 in and the whole first course above that. Follow the manual for your block.
How much gravel do I need behind and under the wall?
Allan Block specifies at least 6 in of compacted wall rock under the first course in a trench 12 in wider than the block depth (24 in for a 12 in block), and wall rock in the block cores and at least 12 in behind the wall. For a 20 ft × 3 ft wall of 12 in deep blocks that is 20 × 2 × 0.5 = 20 cu ft of base plus 20 × 3 × 1 = 60 cu ft behind the wall: 80 cu ft, or 2.96 cu yd, about 4.1 US tons at a typical 1.4 tons per cu yd. Use the gravel calculator for bags and your supplier's density.
How many cap blocks do I need?
Caps run along the top in one row. Divide the wall length by the cap length, round up, and add waste. A 20 ft wall with 16 in caps needs 15, or 17 with 10% extra. Cap lengths differ by product, so enter yours; for example the Allan Block capstone is 18 in long. Caps are sold separately from wall blocks.
Do I need a permit or an engineer for a retaining wall?
Often, once a wall is over about 4 ft. Many US jurisdictions, for example California cities such as Redding and Walnut Creek, require a permit and engineered plans above 4 ft measured from the bottom of the footing to the top of the wall, and lower walls can still need one when they hold up a slope, driveway or building. Versa-Lok lists 4 ft as the maximum unreinforced height of its Standard unit in optimum conditions. This calculator only counts materials, so check local code and ask an engineer about tall walls, poor soil or water.