How it works
A segmental retaining wall is a stack of blocks set on a compacted gravel base. Divide the wall length by the block length to get blocks per course, and the exposed height by the block height to get the courses above ground. Add the buried courses at the bottom, which anchor the wall.
Behind the wall goes free-draining stone and a perforated pipe, so water does not build up pressure on the wall. That stone and the base gravel are ordered by the cubic yard. Add a waste allowance for cuts and breakage, and cap blocks along the top.
Worked example
A 40 ft wall, 3 ft exposed, of 16 × 6 inch blocks with 1 buried course, 5% waste, a 24 inch by 6 inch gravel base, and 12 inches of drainage stone:
- Courses = ⌈36 in ÷ 6 in⌉ + 1 buried = 7
- Blocks per course = ⌈480 in ÷ 16 in⌉ = 30
- Blocks = 7 × 30 × (1 + 5%) = 221
- Base gravel = 40 ft × 2 ft × 0.5 ft ÷ 27 = 1.48 yd³
| Input | Value |
|---|---|
| Wall length | 40 ft |
| Exposed wall height (above grade at the face) | 3 ft |
| Block face length | 16 in |
| Block height | 6 in |
| Buried courses below grade | 1 |
| Cap block length | 16 in |
| Base trench width | 24 in |
| Compacted gravel base depth | 6 in |
| Drainage stone thickness behind the wall | 12 in |
| Waste and breakage allowance | 5 % |
| Result | Value |
|---|---|
| Total courses (including buried) | 7 courses |
| Wall blocks to order (with waste) | 221 blocks |
| Cap blocks to order (with waste) | 32 caps |
| Base gravel (compacted volume) | 1.48 yd³ |
| Drainage stone behind the wall | 5.19 yd³ |
| Perforated drain pipe | 40 ft |
Assumptions and limits
- A simple takeoff. It does not design the wall, choose geogrid, size the embedment, or check soil, drainage, or surcharges.
- Corners, curves, steps, and cuts increase waste. Use a higher allowance for irregular walls.
- Stone volumes are compacted or in-place volumes. Crushed stone is often ordered by the ton: about 1.4 tons per cubic yard for common gravel, but ask your supplier.
- Block dimensions vary by manufacturer. Use the face length and height from the product data.
Common questions
How many retaining wall blocks do I need?
Divide the wall length by the block face length, then multiply by the total number of courses, including buried ones. Add 5–10% for waste. Use the tool above for the exact count.
How high can I build a retaining wall without a permit?
The model building code exempts walls up to 4 feet from the bottom of the footing to the top, unless they support a surcharge, but local rules differ. Check before you build.
How much gravel do I need behind the wall?
A common practice is about 12 inches of clean drainage stone behind the wall, running the full height, with a perforated pipe at the bottom that drains to daylight.
How deep should the first course be buried?
It varies with the block maker's instructions and wall height. Many manufacturers call for at least one course or 6 inches, more for taller walls.
Sources
- Quantity takeoff arithmetic (geometry and 27 ft³ per cubic yard).
- International Building Code Section 105.2: retaining walls not over 4 feet in height (footing bottom to top of wall) are exempt from permit unless supporting a surcharge. Local amendments vary.
Updated 2026-09-30