How Much Do I Need?

Retaining Wall Block Calculator

How many retaining wall blocks do I need?

Enter the wall length and height and the face size of your blocks. The calculator counts courses and blocks per course, adds a buried first course and caps, and works out the gravel for the base and drainage.

Units
Wall

Most block makers call for at least one course below ground.

Blocks

For cut and damaged blocks, and spilled gravel.

Base and drainage gravel

Compacted crushed stone under the first course. Enter 0 to skip.

Width of the clean gravel zone behind the blocks. Enter 0 to skip.

Cost (optional)

Add the price to see an estimated total.

Results update as you type.

Your estimate

Blocks to buy

147 blocks

7 courses of 20, plus 21 caps, including 5% extra.

Estimated amount to buy

Wall blocks
147
Cap blocks
21
Gravel (base + drainage)
2.75 yd³

Calculated quantity (no extra)

Courses (including buried)
7
Blocks per course
20
Blocks
140
Base gravel
0.741 yd³ (20 ft³)
Drainage gravel
1.73 yd³ (46.67 ft³)
How this was calculated
  1. Courses above ground = 2 ft ÷ 4 in = 6, rounded up to 6; plus 1 buried = 7
  2. Blocks per course = 20 ft ÷ 12 in = 20, rounded up to 20
  3. Blocks = 7 × 20 = 140; with 5% extra × 1.05 = 147, rounded up to 147
  4. Caps = 20 ft ÷ 12 in × 1.05, rounded up = 21
  5. Base gravel = 20 ft × 24 in × 6 in = 0.741 yd³ (20 ft³)
  6. Drainage gravel = 20 ft × 2.33 ft tall × 12 in = 1.73 yd³ (46.67 ft³)
  7. Gravel with 5% extra = 2.6 yd³, rounded up to 2.75 yd³
  • Taller retaining walls, often anything over about 3–4 ft (around 1 m), or walls holding back slopes, driveways or buildings, may need an engineered design and a permit. Check local rules.
  • Follow the block maker’s installation guide for setback, geogrid and drainage pipe.

How the retaining wall calculation works

  1. Courses = wall height above ground ÷ block height, rounded up, plus the buried courses.
  2. Blocks per course = wall length ÷ block length, rounded up.
  3. Blocks = courses × blocks per course, plus the extra, rounded up.
  4. Caps = wall length ÷ cap length, plus the extra, rounded up.
  5. Base gravel = wall length × trench width × base depth. Drainage gravel = wall length × full wall height (including buried courses) × drainage width.

Common starting points

The defaults follow typical guidance for small segmental block walls: one buried course, a 6 in (15 cm) compacted gravel base in a trench about twice the block depth, and a 12 in (30 cm) band of clean gravel behind the wall for drainage. Block makers publish exact requirements for their systems, including how far each course steps back, and those take priority.

Rounding rules

  • Courses, blocks per course, blocks and caps are rounded up.
  • Gravel is rounded up to the next 0.25 yd³ or 0.25 m³.
  • Curved walls and corners change the count. Measure along the face of the wall and add a few extra blocks.