🧱 Retaining Wall Calculator
Enter the wall length and height and your block size, and get the blocks per course, the number of courses, cap blocks and the base gravel volume.
Total blocks
200
20/course × 9 courses + 20 caps
Courses high
9
Base gravel
0.56 yd³
Blocks = ⌈length ÷ block width⌉ per course × ⌈height ÷ block height⌉ courses, plus one cap course. Base allows ~6″ (15 cm) of compacted gravel. Walls over ~3–4 ft usually need engineering, geogrid and a permit — check local codes. 🔒 In your browser.
How the retaining wall calculator works
The number of blocks in each course is the wall length divided by the block face width, rounded up. The number of courses is the wall height divided by the block height, rounded up. Multiply the two for the body of the wall, then add one cap course along the top. The tool also estimates a compacted gravel base trench — roughly 6 inches (15 cm) deep and about 1.5 block-widths wide — to sit the first course on.
Segmental block retaining walls taller than about 3–4 feet (1–1.2 m) usually require engineering, geogrid soil reinforcement, drainage aggregate behind the wall and often a permit. Always build on compacted gravel, backfill with free-draining stone, and check your local codes before starting — this tool estimates block quantities only, not structural design.
Frequently asked questions
How many blocks do I need for a retaining wall?
Divide the wall length by the block width and round up for blocks per course, divide the height by the block height and round up for the number of courses, then multiply the two and add a cap course. A 20 ft long, 3 ft high wall of 12 × 4 in blocks needs about 20 per course × 9 courses ≈ 180 blocks plus 20 caps.
How many courses high can I build?
For dry-stacked segmental blocks, walls up to about 3–4 feet are typically DIY-safe with a proper gravel base and drainage. Taller walls need geogrid reinforcement and usually an engineer and permit — the taller the wall, the greater the soil pressure.
How much gravel base do I need?
Plan on a compacted gravel base trench about 6 inches (15 cm) deep and a little wider than the block, running the full length of the wall. This calculator estimates that base volume for you; add free-draining gravel behind the wall separately.
Do I need cap blocks?
Cap blocks finish the top course, hide the hollow cores and are usually glued on with concrete adhesive. They add one course’s worth of blocks; untick the option if your block system doesn’t use separate caps.
Do retaining walls need drainage?
Yes — water pressure behind a wall is a leading cause of failure. Backfill with free-draining gravel and include a perforated drain pipe at the base. This tool covers block quantities; follow the manufacturer’s and local drainage requirements.