◆ Calculators ◆ Retaining Wall Block Calculator: Blocks, Capstones, Corners, Gravel & Geogrid
Retaining Wall Block Calculator: Blocks, Capstones, Corners, Gravel & Geogrid
Work out blocks, capstones, corner blocks, gravel and geogrid rolls for a concrete block retaining wall, sourced from Allan Block's own installation guide.
The total length of the wall, measured along its face.
Walls with an exposed height of 6 ft or more need an engineered design — this calculator covers walls under 6 ft (1.8 m) in height only, per Allan Block's own installation guide.
Allan Block AB Classic block: 8 in (203 mm) high by 18 in (457 mm) long, sold as facing units — change these if your blocks use a different module.
Allan Block AB Classic block: 8 in (203 mm) high by 18 in (457 mm) long, sold as facing units — change these if your blocks use a different module.
Not given directly by the source — derived from Allan Block's own capstone count table (about 0.76 capstones per foot of wall), rounded to a whole-inch length. Change it to match your product.
Number of 90° corners in the wall. A corner needs an extra block in every course, so this scales with the number of courses.
The number of courses that actually step down, not the number of corners — a stepped course needs one extra corner block only in the course where it steps, unlike a true corner, which repeats in every course.
Whether geogrid is needed, and how many layers, depends on soil type and wall height — Allan Block gives this only as a table image, not as text. Enter 0 if you don't know, or the number your engineered design calls for.
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How the number is worked out
The steps from input to result
This calculator turns a wall length and an exposed height into the blocks, capstones, corner blocks, gravel and geogrid rolls a concrete block retaining wall needs, following Allan Block's own Residential Retaining Wall Guide. Allan Block's product specification for its AB Classic block states "11 blk per m2" — for that block's own 200 × 460 mm size, this is exactly wall length ÷ block length, which is why this calculator uses that division rather than the guide's own course-count table on page 59, which gives a different, roughly 32% higher figure for the same block without stating a reason for the gap.
Two separate quantities come from the same source passage on base preparation: "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 1 in. per ft of wall height is the embedment — how far the first course sits below grade — and is added to the exposed height before the course count is worked out. The flat 6 in. figure is not part of the embedment; it belongs to a separate instruction a few lines later: "Place a minimum of 6 in. (150 mm) of wall rock in the base trench and rake smooth." — the gravel bed under the block, counted separately in the gravel figure below, not folded into the block or course count.
Corner blocks follow a different rule from ordinary blocks: "1 for each course that steps down. 1 for each course with a corner." A true corner repeats in every course of the wall, so the corner count here scales with the number of courses; a stepping course, in contrast, only needs its extra block in the one course where the wall actually steps down, so it does not scale with the course count — that asymmetry is why this calculator has two separate fields, "Corners" and "Stepping courses", instead of one.
Gravel is figured in two parts and added together: the base trench, and a drainage zone the source also specifies — "In all cases wall rock is located within the cores of the block and a minimum of 12 in. (300 mm) behind the block." — running the full buried-plus-exposed height of the wall along its length.
Geogrid reinforcement is left as a plain number of layers you enter, because Allan Block gives the decision of whether a wall needs it, and how many layers, only as a table image tied to soil type and wall height, not as text this calculator can read. Once a layer count is entered, the roll count follows the source directly: "To determine the number of rolls needed, multiply the length of your wall (in feet) by the number of layers needed, and then divide by 50 (the length of a roll of AB Reinforcement Grid)."
This calculator only produces a result for walls under 6 ft: "This installation guide focuses on residential retaining walls under 6 ft (1.8 m) in height." A wall at or above that height gets a warning instead of a number, because a wall that tall needs an engineered design this calculator does not attempt to provide.
The formula
Written out, so you can check it by hand
Embedment (in) = 1 × exposed height (ft). Courses = round up((exposed height in in + embedment) ÷ block height). Blocks needed = round up((wall length in in) ÷ block length) × courses. Capstones = round up((wall length in in) ÷ capstone length). Corner blocks = courses × corners + stepping courses. Gravel (cu ft) = (2 ft × 0.5 ft × wall length) + (1 ft × (exposed height + embedment ÷ 12) × wall length), no rounding. Gravel (cu yd) = gravel (cu ft) ÷ 27. Geogrid rolls = round up(wall length × geogrid layers ÷ 50) if geogrid layers > 0, else 0.
Worked example
Worked with the preset values
With a 30 ft wall, a 3 ft exposed height, an 8 × 18 in block and a 16 in capstone: embedment = 1 × 3 = 3 in, so courses = round up((3×12 + 3) ÷ 8) = round up(39 ÷ 8) = round up(4.875) = 5. Blocks needed = round up((30×12) ÷ 18) × 5 = round up(20) × 5 = 20 × 5 = 100. Capstones = round up((30×12) ÷ 16) = round up(22.5) = 23. With no corners and no stepping courses, corner blocks = 5×0+0 = 0. Total buried+exposed height = 3 + 3÷12 = 3.25 ft, so gravel = (2×0.5×30) + (1×3.25×30) = 30 + 97.5 = 127.50 cu ft, or 127.50 ÷ 27 = 4.72 cu yd. With 0 geogrid layers, geogrid rolls = 0.
What this calculator does not do
Named, not left out
- 01This calculator only covers walls with an exposed height under 6 ft — at 6 ft or more it shows a warning instead of a number, because a wall that tall needs an engineered design.
- 02Whether a permit is required — Allan Block notes most municipalities require one for walls over 4 ft — is not checked here; check with your local building department.
- 03A drain pipe can be required for any reinforced wall, gravity walls over 4 ft, or sites with poor drainage; this calculator does not size or count one.
- 04The embedment formula (1 in per ft of exposed height) is Allan Block's own; NCMA's Segmental Retaining Walls Best Practices Guide instead gives a minimum embedment of H'/20, with a floor of 6 in — a different rule from a different source, not blended in here.
- 05The block count formula (wall length ÷ block length) departs from Allan Block's own course-count table on page 59, which gives a roughly 32% higher figure for the same AB Classic block — the source contradicts itself, and this calculator follows the product specification, not the table.
- 06Whether geogrid reinforcement is needed, and how many layers, depends on soil type and wall height, given by the source only as a table image — this calculator leaves it as a number you enter yourself.
- 07Curved or curving walls are not covered by this calculator.
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Updated: 9/18/2026