How it works
The formulas
buried inches = 1 in per ft of height, at least 6 in
blocks per course = ceil(length ÷ block width)
blocks = courses × per course × 1.05, rounded up
caps = ceil(length ÷ cap length)
adhesive tubes = ceil(length ÷ 10 ft)
base stone = 6 in × 24 in × length, + 10%
drainage gravel = 12 in × height × length, + 10%
Assumptions
- The bottom of the wall is buried so it can't slide forward: 1 in per foot of exposed wall height, at least 6 in, which is how segmental block makers such as Allan Block describe it. This is typical practice; we have not checked each maker's current install guide, so read the one for your block. Whole courses are buried, so 4 in blocks get 2 buried courses and 6 in blocks get 1. The buried courses sit on the 6 in crushed stone leveling pad.
- Blocks get 5% extra for cuts and breakage (rule of thumb). The wall height is the part showing above ground; the buried courses are added on top of that.
- Caps are 12 in long by default (edit it for your cap). Caps are glued down; a 10 oz tube of construction adhesive covers about 10 to 12 ft of cap, and we use the low end. Rule of thumb, not from a maker. Pinned, lipped or interlocking blocks need no glue between courses.
- The leveling pad is crushed stone, 6 in deep and 24 in wide, under the first course. Gravel behind the wall is 12 in thick for the full height. These are typical install-guide figures, not checked against every maker.
- Both gravel orders add 10% for compaction. The base is bought by the ton at 1.4 tons per cu yd (ask your supplier for theirs) and rounded up to a half ton. The drainage gravel is bought by the cu yd and rounded up to a half yard.
- Landscape fabric and a 4 in perforated drain pipe are not priced, because they depend on your site. Plan for fabric between the gravel and the soil, and pipe the full length of the wall at the bottom of the gravel, sloping to a daylight outlet.
- Metric: type the wall length in meters. It is converted to feet for the math. Block and cap sizes stay in inches, and gravel is still bought in US tons and cu yd.
Worked example
A 20 ft long, 2 ft high wall of 12 × 4 in blocks with caps
- Courses: 24 in ÷ 4 in = 6.00, so 6 showing; bury 6 in (1 in per ft, 6 in minimum) ÷ 4 in = 2 buried; 8 courses
- Per course: 240 in ÷ 12 in = 20 blocks
- Blocks: 8 × 20 = 160, × 1.05 = 168.0, so 168 blocks
- Block cost: 168 × $6 (the middle of our typical range) = $1,008
- Caps: 240 in ÷ 12 in = 20 caps, with 2 tubes of adhesive
- Base stone: 22.0 cu ft with compaction = 0.81 cu yd × 1.4 = 1.14 tons, so 1.5 tons
- Drainage gravel: 58.7 cu ft with compaction = 2.17 cu yd, so 2.5 cu yd
Common wall sizes, 12 × 4 in blocks
| Length | Blocks, 2 ft high | Blocks, 3 ft high | Caps | Base stone (tons) | Drainage gravel, 2 ft / 3 ft (cu yd) |
|---|---|---|---|---|---|
| 10 ft | 84 | 116 | 10 | 1 | 1.5 / 1.5 |
| 20 ft | 168 | 231 | 20 | 1.5 | 2.5 / 3 |
| 30 ft | 252 | 347 | 30 | 2 | 3.5 / 4.5 |
| 50 ft | 420 | 578 | 50 | 3 | 5.5 / 7.5 |
12 × 4 in blocks, 12 in caps, buried courses (1 in per ft, 6 in minimum) and 5% extra blocks. Heights are above ground.
Questions people ask
How many retaining wall blocks do I need?
Work out the courses (height ÷ block height, rounded up, plus the buried courses), multiply by the blocks in one course (length ÷ block width, rounded up), then add 5%. A 20 ft wall that is 2 ft high with 12 × 4 in blocks is 8 courses of 20, or 168 blocks. With 18 × 6 in blocks the same wall needs 74, and at 3 ft with 12 × 4 in blocks it needs 231.
How deep should the first course be buried?
The common rule, which Allan Block states, is 1 in per foot of exposed wall height, at least 6 in, on a level bed of compacted crushed stone. We bury whole courses: a 4 in block wall up to 3 ft high gets 2 buried courses (8 in), and an 18 × 6 in block gets 1. We have not checked every maker's current guide, so follow the one for your block.
How much gravel do I need behind a retaining wall?
A common rule is 12 in of clean gravel behind the wall, the full height. For a 20 ft wall 2 ft high that is 53 cu ft, or 2.17 cu yd with 10% for compaction, so order 2.5 cu yd. Add the 6 in leveling pad under the wall: 1.5 tons for the same wall.
How tall can I build a retaining wall without an engineer?
Typically up to about 3 to 4 ft, depending on your local code, so check yours before you dig. Anything holding a slope, a driveway or a building, a wall with water running at it, or two walls stacked close together can need an engineer and a permit at any height. Walls over about 3 ft usually need geogrid reinforcement, which this calculator does not plan. This page stops at 3 ft.
Do I need caps and glue?
Caps give a clean, finished top and shed water off the wall. They are glued down with construction adhesive so they don't shift or get knocked off, about one 10 oz tube per 10 ft of cap. A 20 ft wall needs 20 caps and 2 tubes. If your block is a self-capping style, untick the caps box.