Home › Retaining Wall Block Calculator
Retaining Wall Block Calculator
Estimate the materials for a segmental block retaining wall. Enter the length and height of the wall and the size of your blocks, and the calculator returns the number of courses, wall blocks, cap blocks, base gravel, drainage gravel and drain pipe.
Results are estimates. Soil, stone and mulch settle, weigh differently and vary by supplier. Confirm quantities and local requirements before ordering.
Wall blocks per 10 ft of wall (12 × 4 in blocks, one buried course)
| Wall height | Courses | Blocks per 10 ft |
|---|---|---|
| 1 ft | 4 | 40 |
| 2 ft | 7 | 70 |
| 3 ft | 10 | 100 |
| 4 ft | 13 | 130 |
How to calculate it
Divide the wall height by the block height to find the number of courses, rounding up. Add one course for the buried first course below ground. Divide the wall length by the block width to find the blocks per course, then multiply the two.
The base is a compacted gravel trench under the first course, and drainage gravel goes behind the wall. Their volume is length × width × depth, divided by 27 for cubic yards. A perforated drain pipe runs along the base to carry water out.
Worked example
A 20 ft long, 3 ft high wall of 12 × 4 in blocks with a buried first course and 5% extra:
| Courses | 10 9 visible plus 1 buried |
|---|---|
| Blocks per course | 20 |
| Wall blocks to order | 210 200 exact, 5% extra for cuts and breakage |
| Cap blocks | 21 one per 12 in of wall length |
| Base gravel | 0.74 cubic yards about 1 tons of crushed stone, compacted in layers |
| Drainage gravel behind wall | 2.22 cubic yards about 3.1 tons of clean angular stone |
| Perforated drain pipe | 20 ft along the base behind the wall |
| Face area | 60 sq ft |
Tips
- Many areas require a permit or an engineered design for walls over about 3 to 4 feet. Check local rules before you build tall walls.
- Compact the gravel base in thin layers and make sure the first course is level, since every course above follows it.
- Use clean, angular drainage stone behind the wall and keep topsoil out of it so water can drain.
- Order a few extra blocks for cuts at the ends and corners. Check the manufacturer's guide for setback and geogrid requirements.
Assumptions and sources
- Courses = wall height ÷ block height, rounded up, plus one buried course. Blocks = courses × (wall length ÷ block width, rounded up), plus the extra for cuts (default 5%).
- Cap blocks = wall length ÷ cap width. Base gravel = length × base width × base depth ÷ 27; drainage gravel = length × height × thickness ÷ 27; both converted to tons at 1.4 tons per cubic yard.
- Walls over about 3–4 ft often need a permit or engineer. This calculator does not check structural requirements.
- 1 cubic yard = 27 cubic feet = 0.764555 cubic meters.
- 1 US ton = 2,000 lb = 907.185 kg.
- Quantities that are bought whole (bags, boards, pavers, pallets) are rounded up.
Densities and bag sizes are typical supplier and manufacturer figures, not guarantees. Unit conversions follow NIST exact definitions. Full details are on the methodology page. Last reviewed October 2026.
Frequently asked questions
How many blocks do I need for a 20 ft long, 3 ft high wall?
With 12 × 4 in blocks, there are 9 visible courses plus a buried one, which is 10 courses of 20 blocks, or 200 blocks before any extra. A 5% allowance makes it 210.
Do I need to bury the first course?
Yes. Manufacturers generally recommend burying at least the first course so the wall is anchored and level. Follow your block maker's instructions.
How much gravel is needed behind a retaining wall?
Around 12 inches of clean angular stone is typical, plus a drain pipe at the base. This calculator estimates both.
Do I need cap blocks?
Cap blocks finish the top course and are glued in place. This calculator counts one per cap width of wall length, and you can turn them off.