Why Retaining Wall Block Counts Include a Buried Course

The retaining wall calculator counts more block than the visible face of a wall might suggest. That is deliberate. Allan Block’s “Residential Retaining Wall Guide,” retrieved 2026-09-18, says the trench depth includes additional embedment for buried block, so the first course is part of the wall even though backfill will hide it. The calculator therefore starts with the exposed height, adds the buried height required by the guide, and only then works out how many courses are needed.

The hidden course is still a course

Allan Block’s “Residential Retaining Wall Guide,” in its base-preparation instruction, says: “The depth of the trench will be 6 in. (150 mm) plus an additional 1 in. (25 mm) for each 1 ft. (300 mm) of wall height for the amount of buried block that is needed.” The calculator uses the additional part of that instruction as embedment: 1 in. of buried block for every 1 ft. of exposed wall height. In other words, the buried portion is not a miscellaneous allowance added after the block order. It changes the vertical block height that the courses must cover.

That is why the calculator’s course formula is exposed height in inches plus embedment, divided by block height, then rounded up. Allan Block’s source passage describes block that is needed below grade. You buy and lay that course even though you will never see it after the wall is backfilled: the first course sits below grade as part of the wall’s stability arrangement. Leaving that row out would make the visible elevation look right on paper while undercounting the block physically installed in the trench.

The order of operations matters. The calculator adds embedment before dividing by the block height and rounding up. Rounding first on the exposed height alone would discard the buried portion; adding a fractional course afterward would not describe how blocks are actually laid. Courses are whole rows, so the result is rounded up after the combined buried-plus-exposed height has been divided by the selected block height. This explanation follows Allan Block’s embedment instruction and the calculator’s stated course formula in the Allan Block guide’s terms.

Embedment is not the base-rock thickness

The same Allan Block guide gives a separate base-material instruction: “Place a minimum of 6 in. (150 mm) of wall rock in the base trench.” That 6 in. is gravel beneath the block, not 6 in. of additional buried block. The calculator keeps the two quantities separate. Embedment affects the course count because it is block height below grade. The 6-inch base-rock layer affects the gravel volume because it is material in the trench beneath the wall.

This distinction prevents a common geometry mistake. The guide’s quoted trench instruction contains both a flat 6-inch depth and an additional 1 inch for each 1 ft. of wall height, but they serve different purposes in the calculator. The additional inches describe buried block. The flat 6 inches describe the base-rock bed. The calculator does not turn the base-rock thickness into another block course.

The gravel calculation has a second, separate component. Allan Block’s “Water Management” instruction calls for “a minimum of 12 in. (300 mm) behind the block.” The calculator models that drainage zone as 12 in. wide behind the wall, running along the wall and through the full buried-plus-exposed height. It then adds the base-trench volume to the drainage-zone volume.

The height relationship is linear, not superlinear

The calculator’s geometry does not claim that anything grows more than linearly with exposed height. Allan Block’s 1-inch-per-foot embedment rule is proportional to exposed height. The drainage-zone height is the exposed height plus that proportional embedment, so it also changes linearly with exposed height. The base trench uses a flat 6-inch rock thickness across the wall length, so that part is a constant term with respect to height, not an extra height multiplier.

Put another way, the formula contains one fixed base-rock contribution and one drainage contribution whose vertical dimension is the total buried-plus-exposed height. The buried-plus-exposed height is exposed height plus exposed height multiplied by the guide’s 1-inch-per-foot embedment rate. There is no squared-height term and no claim of more-than-linear growth. This is also why the calculator reports gravel as cubic feet and cubic yards without rounding the volume itself: it combines the two stated geometric parts, then converts cubic feet to cubic yards by dividing by the source-backed conversion in its formula.

A worked example from the calculator

The calculator’s example uses a 30 ft wall with a 3 ft exposed height, an 8 × 18 in block, and a 16 in capstone. Allan Block’s source-based example also sets corners to zero, stepping courses to zero, and geogrid layers to zero.

First, the Allan Block embedment rule gives 1 × 3 = 3 in. of buried block. The combined vertical block height is therefore 3 × 12 + 3 = 39 in. The course calculation is round up(39 ÷ 8), or round up(4.875), which gives 5 courses. The buried row is included in those 5 courses; it is not a separate material category that can be ignored when ordering block.

For the block count, the calculator rounds the wall length divided by the block length before multiplying by the course count. Using the example’s dimensions from Allan Block’s source-backed calculator data, round up((30 × 12) ÷ 18) × 5 becomes round up(20) × 5, which equals 20 × 5 = 100 blocks.

Capstones are a separate row along the top, independent of the number of courses below. With the example’s 16 in capstone, round up((30 × 12) ÷ 16) is round up(22.5), giving 23 capstones. Corner blocks use the calculator’s formula of courses × corners plus stepping courses. With 5 courses, no corners, and no stepping courses, that is 5 × 0 + 0 = 0 corner blocks.

For gravel, the total buried-plus-exposed height is 3 + 3 ÷ 12 = 3.25 ft. The Allan Block source-backed calculator separates the base trench from the drainage zone. The base-trench part is 2 × 0.5 × 30 = 30 cu ft: a 24 in trench width represented as 2 ft and a 6 in rock thickness represented as 0.5 ft, across 30 ft of wall. The drainage part is 1 × 3.25 × 30 = 97.5 cu ft: a 12 in zone represented as 1 ft, the full 3.25 ft total height, and the 30 ft wall length.

Adding those two parts gives 30 + 97.5 = 127.50 cu ft, or 4.72 cu yd after the calculator’s conversion. The example has 0 geogrid layers, so geogrid rolls are 0. Each of those figures comes from a different part of the Allan Block-based formula: the buried block affects courses and drainage height, the flat base-rock bed affects gravel, and the entered geogrid-layer count affects rolls.

Why block totals can look inconsistent in the source

Allan Block’s “Residential Retaining Wall Guide” contains two block-count references that do not match. On page 2, the AB Classic product specification says “11 blk per m².” The calculator’s explanation identifies that as the basis for its wall-length-divided-by-block-length approach for the AB Classic block. On page 59, the same guide gives a course-count table for the same block, and the calculator’s explanation says that table is roughly 32% higher. The source gives no reason for the gap.

That is a disclosed, unresolved difference between two places in the same manufacturer document. It is not presented here as an error in the calculator. The calculator follows the product specification used in its stated block-count formula rather than silently blending the page-59 table into the result. Separately, it still uses the guide’s embedment instruction to determine courses, because the buried course is part of the physical wall even when it disappears below grade.

Limits to keep in mind

Allan Block’s guide says its installation guidance focuses on residential retaining walls under 6 ft in height. The calculator returns a warning instead of a number at 6 ft or taller, because those walls need an engineered design. The guide also notes that most municipalities require a permit for walls over 4 ft, but the calculator does not check permitting. Drain-pipe requirements are not sized or counted, and the calculator does not cover curved or curving walls.

Geogrid is also intentionally left as an input. Allan Block gives the decision about whether reinforcement is needed, and how many layers are appropriate, in a table image tied to soil type and wall height rather than in text the calculator can read. Once a layer count is entered, the guide’s roll instruction is to multiply wall length in feet by layers and divide by 50, the length of a roll of AB Reinforcement Grid. For a design that calls for no layers, as in the worked example, the calculator reports zero rolls.

The practical takeaway is simple: read the course count as the number of rows actually installed, not merely the number visible above grade. The retaining wall calculator makes that buried row explicit by applying Allan Block’s 1-inch-per-foot embedment before rounding up the combined height. Its gravel figure then keeps the flat base-rock bed and the 12-inch drainage zone separate, preserving the geometry described by Allan Block’s “Residential Retaining Wall Guide.”

• Retaining Wall Block Calculator: Blocks, Capstones, Corners, Gravel & Geogrid