The formula
Courses = wall height (in) ÷ block height, rounded up, + buried courses Blocks = courses × wall length (in) ÷ block length × (1 + extra %) Caps = wall length (in) ÷ cap length × (1 + extra %) Base = length × 2 ft wide × base depth → cubic yards × 1.4 tons Drainage = length × 1 ft wide × built height → cubic yards × 1.4 tons
Example: a wall 20 ft long and 2 ft high with 12 × 4 inch blocks has 6 courses above ground and 1 buried, so 7 courses of 20 blocks: 147 blocks with 5% extra, and 21 caps. A 6 inch base 24 inches wide is about 1.04 tons of gravel, and 12 inches of drainage stone behind the wall is about 2.42 tons.
Quick answers for common walls
| Wall | Blocks (12 × 4 in face) | Caps | Base gravel | Drainage stone |
|---|---|---|---|---|
| 10 ft long, 1 ft high | 42 | 11 | 0.52 tons | 0.69 tons |
| 20 ft long, 2 ft high | 147 | 21 | 1.04 tons | 2.42 tons |
| 20 ft long, 3 ft high | 210 | 21 | 1.04 tons | 3.46 tons |
| 30 ft long, 2 ft high | 221 | 32 | 1.56 tons | 3.63 tons |
| 50 ft long, 2 ft high | 368 | 53 | 2.59 tons | 6.05 tons |
| 50 ft long, 3 ft high | 525 | 53 | 2.59 tons | 8.64 tons |
How a block wall is built
- Base: a trench about twice the block depth wide, with 6 inches of compacted crushed gravel, and the first course buried.
- Drainage: 12 inches of clean stone behind the blocks, with landscape fabric between the stone and the soil. Taller walls and wet soil also need a perforated drain pipe at the base.
- Setback: most blocks have a lip that steps each course back slightly, which helps the wall lean into the hill.
- Height limits: the IRC requires an engineered design for walls holding back more than 4 ft of soil, or more than 2 ft when they also carry other loads such as a driveway or a slope above. Many block makers require geogrid reinforcement at lower heights; follow their installation guide.
Sources and assumptions
Block faces are typical sizes for segmental retaining wall blocks; check the size printed on your block and change Block face if needed. Gravel uses 1.4 tons per cubic yard. Height limits from the 2021 International Residential Code, section R404.4.
Questions
How many retaining wall blocks do I need?
Courses times blocks per course. A 20 ft wall 2 ft high with 12 × 4 inch blocks has 6 courses above ground plus 1 buried, 20 blocks each: 140 blocks, or 147 with 5% extra.
How much gravel goes behind a retaining wall?
A zone about 12 inches wide for the full height of the wall. For a 20 ft wall 2 ft high that is about 1.7 cubic yards, or 2.42 tons, of clean drainage stone.
How many blocks should be buried?
At least one course, and more for taller walls. The block maker’s installation guide gives the depth for each wall height.
When does a retaining wall need an engineer?
Under the IRC, when it holds back more than 4 ft of soil, or more than 2 ft while also carrying other loads such as a driveway or a slope above. Local rules can be stricter, and many towns require a permit for taller walls.
References and review
- International Residential Code 2021, chapter 4 (section R404.4: retaining walls)
- Call 811: free utility line marking before you dig
- NIST Special Publication 811: unit conversion definitions
Units convert with exact definitions: 1 ft = 12 in = 0.3048 m, 1 yd³ = 27 ft³. Results round up to whole bags, sheets and pieces so you don’t come up short. Last reviewed October 20, 2026. If your bag, pallet or supplier lists a different figure, use theirs: change the matching field above.