How it works
The calculator works out how many courses the wall needs, how many blocks fill one course, and multiplies them. Caps are counted along the wall length.
Blocks per course = ⌈ Wall length ÷ Block face length ⌉
Courses = ⌈ (Exposed height + Buried depth) ÷ Block height ⌉
Blocks = ⌈ Courses × Blocks per course × (1 + Waste ÷ 100) ⌉
Caps = ⌈ Wall length ÷ Cap length × (1 + Waste ÷ 100) ⌉
⌈ ⌉ means rounding up to the next whole number. For a 20 ft wall of 18 × 8 in blocks that shows 3 ft and is buried 6 in, blocks per course are 240 ÷ 18 = 13.3, rounded up to 14. Courses are (36 + 6) ÷ 8 = 5.25, rounded up to 6. That is 6 × 14 = 84 blocks and 14 caps.
Where the defaults come from
The block and cap sizes are from Allan Block’s product page for the AB Collection. It lists the AB Classic at 8 in (200 mm) high and 18 in (460 mm) long, about 1 sq ft of face per block, and the AB Capstone at 4 in (100 mm) high and 18 in (460 mm) long. Allan Block also notes that all specifications are approximate. Other systems use other sizes, so replace the defaults with the face length and height on your block’s product sheet.
The buried depth default is from Allan Block’s gravity wall construction guide, which says buried block should be a minimum of 6 in (150 mm) and to check plans for how much is required. Installation guides differ in how they set the buried depth, and your plans may require more than the minimum. That is why it is an input, not a fixed number.
Why the buried course matters
The buried part of the wall is real block that you have to buy. Because blocks come in whole courses, the buried depth often adds a full course to the count. In the example, 3 ft of exposed wall alone would be 36 ÷ 8 = 4.5 courses, so 5. With 6 in buried, the total is 42 in, which needs 6 courses. The courses output shows the total, including the buried one.
Height limits and engineering
A segmental retaining wall holds back soil by its own weight and setback up to a certain height. Above that, it needs soil reinforcement such as geogrid and an engineered design. Allan Block publishes a gravity wall chart showing the maximum height before reinforcement is required. The height varies by block type, soil type and the slope or load above the wall, and the chart does not account for seismic loading. Allan Block states that final designs for construction purposes must be performed by a local registered professional engineer.
This calculator does not check any of that. It counts blocks for the height you enter. If your wall is near or above the unreinforced height your manufacturer states, or if there is a slope, driveway, building or other load above it, get an engineered design before you buy material. Your local building department may also require a permit or engineering, so check with them before you start.
Drainage, base and backfill
Allan Block’s installation guide also calls for a compacted base, wall rock in and behind the blocks, and a drain pipe for walls over 4 ft (1.2 m) tall or built in silty or clay soils. None of those are counted here. Use the gravel calculator for base and drainage aggregate, and follow the product’s installation guide for depths and widths.
Walls that are not a single straight run
For walls with corners, curves or stepped sections, split the wall into straight sections with a single height each, run the calculator for each one, and add the results. Corner units, step-up blocks and cut caps on curves are planned from the manufacturer’s guide. Add a waste allowance if you expect to cut many blocks.
Worked example
Inputs
- Wall length
- 20 ft
- Exposed wall height
- 3 ft
- Buried depth
- 6 in
- Block face length
- 18 in
- Block height
- 8 in
- Cap length
- 18 in
- Waste allowance
- 0 %
Result
- Exposed wall face
- 60 sq ft
- Blocks per course
- 14
- Courses incl. buried
- 6
- Wall blocks
- 84
- Cap units
- 14
Common mistakes
- Forgetting the buried course. The first course sits partly or wholly below grade, so a wall that shows 3 ft needs more than 3 ft of block.
- Measuring the cap into the exposed height. Enter the block height that shows, then count caps separately.
- Using the nominal size from a store tag instead of the face length and height on the manufacturer’s product sheet.
- Treating the block count as a design. The safe height without reinforcement depends on the block, soil, slope and loads above the wall – get it from the manufacturer’s chart or an engineer.
- Ignoring base and backfill materials. Base gravel, drainage aggregate, drain pipe, fabric and geogrid are separate items in the installation guide.
Assumptions and limits
- The wall is straight and runs at one height along its full length. Corners, curves and stepped bases or tops are not modeled – split the wall into sections and add them up.
- Blocks per course is the wall length divided by the block face length, rounded up. Dry-stacked units have no mortar joint.
- Courses are the exposed height plus the buried depth divided by the block height, rounded up to a whole course.
- The built-in setback of each course is ignored for the count. Blocks and caps are rounded up after the waste allowance.
- This is a material estimate only. Reinforcement, drainage, base and wall height limits come from the manufacturer’s installation guide, local code or a qualified engineer.
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, then multiply by the number of courses, including the buried one. A 20 ft wall of 18 × 8 in AB Classic blocks that shows 3 ft and has 6 in buried needs 14 blocks per course and 6 courses, so 84 blocks.
How deep should the first course be buried?
Follow the installation guide for your block. Allan Block says buried block should be a minimum of 6 in (150 mm) and to check your plans for how much is required. Other manufacturers state their own rules.
How tall can I build a retaining wall without reinforcement?
It depends on the block system, soil type, the slope above the wall and any loads near it. Allan Block publishes a gravity wall chart that shows the maximum height before reinforcement is required for each of its block types and soil conditions. Above that height, the wall needs geogrid reinforcement and an engineered design.
How many caps do I need?
Divide the wall length by the cap length and round up. With 18 in caps, a 20 ft wall needs 240 ÷ 18 = 13.3, so 14 caps. Caps on curves or corners may need cutting.
Does this include corner blocks?
No. Outside corners usually use special corner units, and steps in the wall change the count. Plan corners with the manufacturer’s installation guide and adjust the order.
Sources
- AB Collection – Retaining Wall Products — Allan Block Corporation. AB Classic: "8 in H (200 mm) … 18 in L (460 mm) … 1 sq ft approx."; AB Capstone: "4 in H (100 mm) … 18 in L (460 mm)". "All specifications are approximate."
- Gravity Retaining Wall Construction — Allan Block Corporation. "Buried block should be a minimum of 6 in (150 mm). Check plans to see how much buried block is required." "Drain pipe is required for walls over 4 ft (1.2 m) tall or are constructed in silty or clay soils."
- Gravity Retaining Walls – Maximum Wall Heights — Allan Block Corporation. "Use the chart below to find the maximum height that your retaining wall can be built before reinforcement is required." Heights depend on soil and the slope above; "Final designs for construction purposes must be performed by a local registered professional engineer".
- NIST Handbook 44, Appendix C – General Tables of Units of Measurement — National Institute of Standards and Technology. Exact unit definitions: 1 in = 2.54 cm, 1 yd³ = 27 ft³, 1 US gal = 231 in³, 1 lb = 0.45359237 kg.
Method reviewed November 3, 2026