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Retaining Wall Block Calculator

Estimates segmental (dry-stacked) retaining-wall blocks from the wall length and height and the block face size (courses × blocks per course plus waste), cap units for the top course, the compacted gravel base under the first course, the drainage-gravel zone behind the wall and the perforated drain pipe length. Planning estimate only.

Wall length (ft or m)Length of the wall face (ft or m, see unit). Wall height (ft or m)Total block height including the buried first course (bury about 10 % of the height, at least 15 cm / 6 in) and excluding the caps. Length unit (optional)Unit of the length inputs (m or ft). Lengths are reported in both units. Block length (in) (optional)Face length of one block in inches (1 in = 25.4 mm); common units are 12 in and 18 in long. Block height (in) (optional)Height of one block in inches (4, 6 or 8 in). Block depth (in) (optional)Front-to-back depth of the block in inches; the base trench is 12 in wider than this. Cap blocks (optional)Add a course of cap units (same length as the blocks) on top of the wall. Base gravel depth (in) (optional)Compacted crushed-stone base under the first course, in inches (6 in / 15 cm is the usual minimum). Drainage backfill depth (in) (optional)Thickness of the clean-gravel drainage zone directly behind the blocks, in inches (12 in / 30 cm is standard). Waste allowance (%) (optional)Extra material for cuts, breakage and offcuts, applied as a multiplier (1 + waste_percent/100). 5 % for straight walls; 10 % for curves and corners that need cut blocks.

When to use

You need to order concrete retaining-wall blocks, caps and the gravel for the base and drainage zone of a garden or landscape wall.

Do not use when: The wall is higher than about 1.2 m (4 ft), carries a slope or surcharge, or is terraced: it needs geogrid and engineering per the block manufacturer's tables. Mortared brick walls use brick; poured footings use concrete-footing.

Formula

courses = ceil(wall_height / block_height); blocks_per_course = ceil(wall_length / block_length); total_blocks = ceil(courses × blocks_per_course × (1 + waste/100)); caps = blocks_per_course; base_gravel = wall_length × (block_depth + 12 in) × base_depth; backfill_gravel = wall_length × wall_height × backfill_depth; tonnes = m³ × 1.6; drain_pipe = wall_length.

Follows the usual segmental retaining-wall detail: a compacted crushed-stone base 6 in deep in a trench 6 in wider than the block on each side, a perforated drain pipe at the base behind the blocks and 12 in of clean gravel backfill; gravel volumes are as-placed (loose material compacts 10–20 %). Planning estimate only; check quantities against the supplier's coverage figures and local code before ordering.

Inputs

Parameter	Type	Unit	Required	Description
`wall_length`	number	ft or m	yes	Length of the wall face (ft or m, see unit). Range: > 0, ≤ 10000
`wall_height`	number	ft or m	yes	Total block height including the buried first course (bury about 10 % of the height, at least 15 cm / 6 in) and excluding the caps. Range: > 0, ≤ 100
`unit`	enum: m \| ft		default ft	Unit of the length inputs (m or ft). Lengths are reported in both units.
`block_length_in`	number	in	default 12	Face length of one block in inches (1 in = 25.4 mm); common units are 12 in and 18 in long. Range: > 0, ≤ 48
`block_height_in`	number	in	default 4	Height of one block in inches (4, 6 or 8 in). Range: > 0, ≤ 24
`block_depth_in`	number	in	default 8	Front-to-back depth of the block in inches; the base trench is 12 in wider than this. Range: > 0, ≤ 48
`cap_blocks`	boolean		default true	Add a course of cap units (same length as the blocks) on top of the wall.
`base_depth_in`	number	in	default 6	Compacted crushed-stone base under the first course, in inches (6 in / 15 cm is the usual minimum). Range: ≥ 0, ≤ 24
`gravel_backfill_depth_in`	number	in	default 12	Thickness of the clean-gravel drainage zone directly behind the blocks, in inches (12 in / 30 cm is standard). Range: ≥ 0, ≤ 48
`waste_percent`	number	%	default 5	Extra material for cuts, breakage and offcuts, applied as a multiplier (1 + waste_percent/100). 5 % for straight walls; 10 % for curves and corners that need cut blocks. Range: ≥ 0, ≤ 50

Outputs

Output	Type	Unit	Description
`courses`	integer		ceil(wall_height / block_height).
`blocks_per_course`	integer		ceil(wall_length / block_length).
`blocks_before_waste`	integer		courses × blocks_per_course.
`total_blocks`	integer		ceil(blocks_before_waste × (1 + waste_percent/100)).
`cap_blocks_needed`	integer		blocks_per_course when cap_blocks is true, otherwise 0.
`base_gravel_yd3`	number	yd³	wall_length × (block_depth + 12 in) × base_depth.
`base_gravel_m3`	number	m³	Base gravel in cubic metres.
`backfill_gravel_yd3`	number	yd³	wall_length × wall_height × gravel_backfill_depth.
`backfill_gravel_m3`	number	m³	Backfill gravel in cubic metres.
`gravel_total_tonnes`	number	t	(base + backfill) × 1.6 t/m³.
`drainage_pipe_ft`	number	ft	Perforated pipe along the back of the base course = wall_length.
`drainage_pipe_m`	number	m	Drain pipe length in metres.

Example

20 ft × 2 ft wall, 12 × 4 × 8 in blocks, caps, 6 in base, 12 in backfill, 5 % waste: {"wall_length":20,"wall_height":2,"unit":"ft","block_length_in":12,"block_height_in":4,"block_depth_in":8,"cap_blocks":true,"base_depth_in":6,"gravel_backfill_depth_in":12,"waste_percent":5} → {"courses":6,"blocks_per_course":20,"blocks_before_waste":120,"total_blocks":126,"cap_blocks_needed":20,"base_gravel_yd3":0.617,"base_gravel_m3":0.472,"backfill_gravel_yd3":1.481,"backfill_gravel_m3":1.133,"drainage_pipe_ft":20,"drainage_pipe_m":6.1}

10 m × 0.8 m wall, 12 × 4 × 8 in blocks, caps, 5 % waste: {"wall_length":10,"wall_height":0.8,"unit":"m","waste_percent":5} → {"courses":8,"blocks_per_course":33,"blocks_before_waste":264,"total_blocks":278,"cap_blocks_needed":33,"base_gravel_m3":0.774,"base_gravel_yd3":1.013,"backfill_gravel_m3":2.438,"backfill_gravel_yd3":3.189,"drainage_pipe_ft":32.81}

GET https://tttkmbb.com/api/v1/calculate/retaining-wall-blocks?wall_length=20&wall_height=2&unit=ft&block_length_in=12&block_height_in=4&block_depth_in=8&cap_blocks=true&base_depth_in=6&gravel_backfill_depth_in=12&waste_percent=5

Machine access

API: GET https://tttkmbb.com/api/v1/calculate/retaining-wall-blocks (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/retaining-wall-blocks · Markdown: https://tttkmbb.com/construction/retaining-wall-blocks.md · JSON definition: https://tttkmbb.com/construction/retaining-wall-blocks.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="retaining-wall-blocks"
OpenAPI operationId: estimate_retaining_wall_blocks
Freshness: static. Authentication: none. Rate limit: fair use (see rate limits).

Sources

Wikipedia – Retaining wall (reference)
Wikipedia – Bulk density (reference)

FAQ

Should the first course be buried?

Yes: bury about 10 % of the wall height, at least one course or 15 cm (6 in), so enter the total height including the buried course.

Why is the base trench wider than the block?

The compacted base extends about 15 cm (6 in) in front of and behind the block to spread the load and keep the base course from shifting, hence block_depth + 12 in.

When do I need an engineer?

Most block systems allow gravity walls up to about 1.2 m (4 ft) with level backfill; taller walls, slopes above the wall, driveways or terraces behind it need geogrid reinforcement designed from the manufacturer's charts or by an engineer, and many jurisdictions require a permit above 1 m.

Related calculators

Gravel Calculator — Aggregate volume and weight for other layers.
Brick Calculator — Mortared brick wall quantities.
Concrete Footing Calculator — Concrete footing for a mortared wall.