Retaining wall block calculator
Enter the length and exposed height of your wall and the face size of the block you plan to use. You get the block count, cap count, and gravel for the base and drainage zone.
Estimates round up. Check product labels and supplier figures before ordering.
Common block sizes
| Block type | Face (H x L) | Typical max height |
|---|---|---|
| Small garden block | 4 x 11.75 in | about 2-3 ft |
| Medium wall block | 6 x 16-18 in | about 3-4 ft |
| Large wall block | 8 x 18 in | about 4 ft, more with geogrid |
| Cap block | 3-4 in tall, 12-16 in long | Top course only |
How it's calculated
The calculator counts blocks course by course, rounding up so each row is complete.
Blocks per course = wall length (in) / block face length (in), rounded up
Courses = exposed height (in) / block face height (in), rounded up, plus 1 buried course
Total blocks = blocks per course x courses
Caps = wall length (in) / cap length (in), rounded up
Example: a wall 20 ft long and 2 ft high built from 4 x 11.75 in blocks needs 240 / 11.75 = 20.4, so 21 blocks per course. Two feet is 24 inches, or 6 courses, plus 1 buried course makes 7. That is 147 blocks and 20 caps of about 12 inches.
Gravel is estimated for a drainage zone 12 inches deep behind the full block height and a leveling pad 6 inches deep and 24 inches wide. Use clean 3/4 in angular stone behind the wall and a compactable base under it. Block sizes differ by brand, so measure the face of the block you will buy.
What you'll need
- Retaining wall blocks
- Retaining wall caps
- Masonry adhesive
- Drainage gravel
- Landscape fabric
Common questions
How many retaining wall blocks do I need?
Divide the wall length by the block length for blocks per course, then multiply by the number of courses. A 20 ft wall of 4 x 11.75 in blocks, 2 ft high plus a buried course, needs about 147 blocks.
How deep should the first course be buried?
A common guideline is to bury at least one full course, or about 1 inch for every 8 inches of wall height, whichever is more. Follow your block maker's instructions.
Do I need gravel behind a retaining wall?
Yes. About 12 inches of clean angular stone behind the blocks lets water drain instead of building pressure that pushes the wall over.
How tall can a block retaining wall be without reinforcement?
Small garden blocks are usually limited to about 2 to 3 feet. Many codes require engineering or a permit for walls over 4 feet.