# How many retaining wall blocks?

Retaining wall calculator: the blocks, courses and cap blocks for a segmental block retaining wall, and the gravel for its base and the drainage zone behind it, with cost.

- Page: https://www.acalculator.org/construction/retaining-wall-calculator
- JSON spec: https://www.acalculator.org/construction/retaining-wall-calculator.json
- Version: e224b2da7c7e

## Default answer

Example with the default inputs (Wall length 20 ft, Wall height 3 ft, Block length 16 in, Block height 6 in, Cap length 16 in, Base trench width 24 in, Base depth 6 in, Drainage zone 12 in): You need 90 wall blocks (6 courses of 15), 15 caps and 2.96 yd³ of gravel.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| l | Wall length | The length of the wall along its face. |
| h | Wall height | The full height of the block courses, the buried course included, caps left out. |
| bl | Block length | The face length of one block. |
| bh | Block height | The height of one block. |
| cl | Cap length | The face length of one cap block. |
| p | Price per block | The price of one wall block. Leave empty to skip the cost. |
| tw | Base trench width | The width of the gravel base under the wall. |
| td | Base depth | The depth of compacted gravel under the first course. |
| dw | Drainage zone | The width of drainage gravel behind the wall, for its full height. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| blocks | Wall blocks | Courses × blocks per course. |
| courses | Courses | Wall height ÷ block height, rounded up. |
| perCourse | Blocks per course | Wall length ÷ block length, rounded up. |
| caps | Cap blocks | Wall length ÷ cap length, rounded up. |
| face | Wall face | Wall length × wall height. |
| base | Base gravel | Length × base trench width × base depth. |
| drain | Drainage gravel | Length × drainage zone × wall height. |
| gravel | Gravel in all | Base gravel + drainage gravel. |
| gravelM3 | Gravel in all, cubic metres | The same in m³. |
| cost | Block cost | Wall blocks × the price per block. |

## Method

Courses = height ÷ block height and blocks per course = length ÷ block length, each rounded up; blocks = courses × blocks per course; caps = length ÷ cap length, rounded up; gravel = length × (base width × base depth + drainage width × height).

## Assumptions

- A straight wall of one block thickness, laid in running courses; the wall height includes the buried first course.
- The defaults (16 × 6 in blocks, a 24 in wide, 6 in deep base, 12 in of drainage gravel) are starting points: use your block maker’s sizes and installation guide.
- A tall wall, or one holding back a slope, a drive or a building, may need an engineer’s design; check local rules.

## Worked examples

1. l = 6.096, h = 0.9144, bl = 0.4064, bh = 0.1524, cl = 0.4064, tw = 0.6096, td = 0.1524, dw = 0.3048, p = $3.50 gives courses = 6, perCourse = 15, blocks = 90, caps = 15, base = 0.740741, drain = 2.222222, cost = $315.00. Source: NIST SP 811 (2008), Appendix B: 1 in = 0.0254 m, 1 yd³ = 27 ft³, https://www.nist.gov/pml/special-publication-811 (retrieved 2026-10-03): 36 ÷ 6 = 6 courses.
2. l = 3.048, h = 0.762, bl = 0.3048, bh = 0.1016, cl = 0.3048, tw = 0.6096, td = 0.1524, dw = 0.3048 gives courses = 8, perCourse = 10, blocks = 80, caps = 10, gravel = 1.296296. Source: NIST SP 811 (2008), Appendix B: 1 in = 0.0254 m, 1 yd³ = 27 ft³, https://www.nist.gov/pml/special-publication-811 (retrieved 2026-10-03): 30 ÷ 4 = 7.5, so 8 courses.
3. l = 6, h = 1, bl = 0.4, bh = 0.2, cl = 0.5, tw = 0.6, td = 0.15, dw = 0.3 gives blocks = 75, caps = 12, gravelM3 = 2.34. Source: NIST SP 811 (2008), Appendix B: 1 in = 0.0254 m, 1 yd³ = 27 ft³, https://www.nist.gov/pml/special-publication-811 (retrieved 2026-10-03): 1 ÷ 0.2 = 5 courses × 6 ÷ 0.4 = 15 = 75 blocks.

## FAQ

### How many blocks do I need for a retaining wall?

Divide the wall height by the block height for the courses, and the wall length by the block length for the blocks in each course, rounding each up, then multiply. A 20 ft wall 3 ft high in 16 × 6 in blocks is 6 courses × 15 blocks = 90 blocks.

### Does the wall height include the buried course?

Yes. Type the full height of all the courses, including the first course set below ground, but not the caps.

### How many cap blocks do I need?

Divide the wall length by the face length of one cap and round up. 20 ft of wall with 16 in caps takes 15 caps.

### How much gravel does a retaining wall need?

Gravel for the base trench (length × trench width × base depth) plus gravel behind the wall for drainage (length × zone width × wall height). A 20 ft wall, 3 ft high, with a 24 × 6 in base and 12 in of drainage gravel needs 20 + 60 = 80 ft³, about 2.96 yd³.

### What if the height is not a whole number of blocks?

The courses round up, so a 30 in wall in 4 in blocks takes 8 courses (32 in). Bury the extra or cut the top course.

### When does a retaining wall need an engineer?

A tall wall, or one holding back a slope, a driveway or a building, may need an engineered design and a permit. Check local rules and the block maker’s height limits.

## Sources

- NIST Special Publication 811 (2008), Appendix B: 1 in = 0.0254 m and 1 ft = 0.3048 m exactly, 1 yd³ = 27 ft³, retrieved 2026-10-03. https://www.nist.gov/pml/special-publication-811
- Wikipedia, Retaining wall: short landscaping walls are often made of segmental concrete units, and proper drainage behind the wall limits the pressure on it, retrieved 2026-10-03. https://en.wikipedia.org/wiki/Retaining_wall