How the retaining wall calculator works
Segmental retaining wall blocks are laid without mortar, so the count is based on the block's face size alone:
blocks per course = ⌈ wall length ÷ block face length ⌉
blocks = courses × blocks per course × (1 + waste %)
cap blocks = ⌈ wall length ÷ cap length ⌉ (+5% for cuts)
Example: a 20 ft wall standing 2 ft above ground with standard 16 × 6 in face blocks needs 4 exposed courses plus 1 buried = 5 courses of 15 blocks = 75 blocks, or 79 with 5% waste, plus about 21 cap blocks (12 in caps).
Bury the first course
The first course sits below ground on a compacted gravel base. Most block makers recommend burying at least one full course, or about 1 in of block for every 8 in of wall height (roughly 10% of the height) — whichever is more. Set Buried courses to 2 for taller walls or walls on a slope.
Drainage gravel, base and pipe
Water pressure behind the wall is the most common reason retaining walls fail. The calculator estimates:
- Drainage aggregate — a 12 in (300 mm) zone of clean ¾ in crushed stone behind the blocks, for the full built height.
- Base pad — a 6 in (150 mm) layer of compacted crushed stone, about 6 in wider than the block on each side.
- Perforated drain pipe — 4 in (100 mm) pipe along the back of the base, outlet to daylight, one length per wall length.
Gravel is shown in cubic yards (or m³) and tons (or tonnes), including 10% for compaction. Wrap the drainage zone in filter fabric if your soil is fine or clay-rich.
Retaining wall block sizes
| Block type | Typical face | Typical depth | Blocks per ft² of face |
|---|---|---|---|
| Small garden wall block | 12 × 4 in | 7 in | 3.0 |
| Medium block | 12 × 6 in | 8 in | 2.0 |
| Standard block | 16 × 6 in | 12 in | 1.5 |
| Large block | 18 × 8 in | 12 in | 1.0 |
Sizes vary by manufacturer, so measure your chosen block or check the product sheet and use Custom size.
Safety: Walls taller than about 3–4 ft (1–1.2 m), walls holding up a driveway, slope or building, and tiered walls usually need geogrid reinforcement and an engineered design, and often a building permit. This calculator is for material estimates only.
Frequently asked questions
How many retaining wall blocks do I need?
Divide the wall length by the block face length to get blocks per course, then multiply by the number of courses — the exposed height divided by the block height, plus at least one buried course. Add 5% for waste.
How many retaining wall blocks are in a square foot?
About 1.5 standard 16 × 6 in face blocks per square foot of wall face, 2 blocks for 12 × 6 in faces and 3 for small 12 × 4 in garden blocks.
How much gravel do I need behind a retaining wall?
Allow a 12 in wide zone of clean crushed stone behind the wall for its full height, plus a 6 in compacted base. For a 20 ft × 2 ft wall that is about 2.9 cubic yards (roughly 4 tons) in total, including compaction.
Do I need cap blocks?
Caps finish the top course and protect it from water. They are usually glued down with masonry adhesive. One cap per cap length of wall, plus a few spare for cuts.
How tall can a block retaining wall be without reinforcement?
Gravity walls with standard blocks are usually limited to about 3–4 ft (1–1.2 m) depending on the block and soil. Taller walls need geogrid and an engineered design — check the block maker's guidelines and local code.