Retaining Wall Block Calculator
Segmental block walls are simple math: blocks per course × courses, plus one buried course. The base and drainage stone are what make a wall last, so this calculator includes them.
Formulas
Blocks per course = wall length (in) ÷ block face length, rounded up. Courses = height ÷ block height, rounded up, + 1 buried course. Base trench is 24 in wide by 6 in of compacted crushed stone. The drainage zone is 12 in of clean ¾ in stone behind the wall, full height.
16 x 6 in block walls with caps
| Wall | Courses | Blocks | Caps | Base stone | Drain stone |
|---|---|---|---|---|---|
| 10 ft × 12 in | 3 | 26 | 10 | 0.64 t | 0.78 t |
| 20 ft × 18 in | 4 | 63 | 20 | 1.28 t | 2.07 t |
| 20 ft × 24 in | 5 | 79 | 20 | 1.28 t | 2.59 t |
| 30 ft × 24 in | 5 | 121 | 30 | 1.92 t | 3.89 t |
| 40 ft × 30 in | 6 | 189 | 40 | 2.56 t | 6.22 t |
Before you build
- Most DIY block systems are rated to about 3–4 ft. Taller walls, and walls carrying a driveway or a slope, usually need engineering and a permit.
- Bury the first course at least 1 in for every 8 in of wall height (minimum 1 course).
- Check the manufacturer's spec sheet for exact face dimensions. Many "12 in" blocks are actually 11.5–12 in.
Frequently asked questions
How many retaining wall blocks do I need?
Divide wall length in inches by block length to get blocks per course, then multiply by courses (height ÷ block height, + 1 buried). For a 20 ft × 24 in wall in 16×6 blocks, that's 15 × 5 = 75, plus 5% = 79.
How much gravel behind a retaining wall?
A 12 in wide zone of clean drainage stone the full height of the wall. For a 20 ft wall 30 in tall in total, about 1.9 yd³.
How deep should a retaining wall base be?
Usually 6 in of compacted crushed stone in a trench twice as wide as the block, with the first course buried.
Last reviewed October 2026 by Jim Frith. These are planning estimates; confirm quantities with your supplier and follow local codes.