How to estimate a block retaining wall
- Measure the length along the face of the wall, and the height you want above finished grade.
- Enter your block size. The face length and height are printed on the product listing — common segmental blocks are around 16–18 in long and 4–8 in high.
- Bury the first course. Most manufacturers recommend burying the base course (about 1 in of buried depth per 8 in of wall height, with a minimum of about 6 in). One buried course is typical for short walls.
- Add caps if you want a finished top, and a small waste allowance for cut blocks at the ends and corners.
Retaining wall formulas
Courses = Height (in) ÷ Block height (in), rounded up + buried courses
Blocks = Blocks per course × Courses
Caps = Wall length (in) ÷ Cap length (in)
Worked example
A 30 ft wall standing 3 ft above ground, using 16 × 6 in blocks: 360 ÷ 16 = 23 blocks per course; 36 ÷ 6 = 6 visible courses plus 1 buried = 7 courses. That's 161 blocks before waste, plus 23 caps.
Block quantity chart
16 × 6 in blocks, one buried course, no waste:
| Wall (length × height) | Blocks / course | Courses | Total blocks | Caps |
|---|---|---|---|---|
| 20 ft × 2 ft | 15 | 5 | 75 | 15 |
| 20 ft × 3 ft | 15 | 7 | 105 | 15 |
| 30 ft × 3 ft | 23 | 7 | 161 | 23 |
| 40 ft × 3 ft | 30 | 7 | 210 | 30 |
| 50 ft × 4 ft | 38 | 9 | 342 | 38 |
Base and drainage
Most retaining walls that fail do so because of water pressure behind them, not the blocks. A typical gravity-wall build includes:
- A leveling base of compacted crushed stone about 6 in deep and roughly 12 in wider than the block depth.
- Drainage aggregate — clean ¾ in crushed stone — filled at least 12 in behind the blocks for the full height.
- A perforated drain pipe at the base of the drainage stone, outletting to daylight.
- Filter fabric between the drainage stone and the native soil to keep fines out.
The calculator estimates both gravel volumes; use the gravel calculator to convert them to tons.
Height limits and permits
In many US jurisdictions, retaining walls over 4 ft — measured from the bottom of the footing — need a permit and a design by a licensed engineer, as do shorter walls holding back a slope, a driveway or a building. Taller walls also typically need geogrid reinforcement layered into the backfill. Check local rules before you dig.
Frequently asked questions
How many retaining wall blocks do I need?
Divide the wall length in inches by the block length to get blocks per course, divide the height in inches by the block height (plus any buried courses) to get the number of courses, and multiply. Add about 5% for cuts.
How many blocks are in a 3 ft tall wall?
With 6 in high blocks, a 3 ft wall has 6 visible courses plus one buried — 7 courses. Each 10 ft of wall with 16 in blocks needs 8 blocks per course, so about 56 blocks per 10 ft.
How much gravel do I need behind a retaining wall?
Plan on a 12 in wide zone of drainage stone for the full wall height. For a 30 ft wall with 7 courses of 6 in block (3.5 ft), that is about 3.89 cubic yards, plus a 6 in leveling base.
How deep should the first course be buried?
A common guideline is 1 in of embedment for every 8 in of wall height, with a minimum of about 6 in — in practice, one full course for most short walls. Follow your block manufacturer's guide.
Do I need a permit for a retaining wall?
Often for walls over 4 ft tall (measured from the bottom of the footing) or any wall supporting a surcharge such as a driveway, slope or structure. Requirements vary — check with your local building department.