The Retaining Wall Calculator calculates the materials for a segmental block retaining wall: wall blocks, cap units, base gravel and drainage stone.
Blocks are counted course by course: blocks per course (wall length ÷ block face length) times the number of courses (exposed height ÷ block height, plus an optional buried course). Gravel volumes come from the trench and drainage-zone dimensions and are shown in cubic yards or cubic meters.
Retaining Wall Calculator
Blocks, cap units, base gravel and drainage stone for a segmental block retaining wall.
Formula
- Blocks per course: ⌈L ÷ block face length⌉
- Courses: ⌈H ÷ block height⌉ + buried course (0 or 1)
- Wall blocks: courses × blocks per course
- Cap units: ⌈L ÷ cap length⌉
- Base gravel: L × trench width × gravel depth
- Drainage stone: L × total wall height × drainage width
Where L = wall length and H = exposed wall height. Total wall height includes the buried course.
How the Calculation Works
Step 1: Work out how many blocks fit along the wall and how many courses reach the height.
Step 2: Multiply to get the block count, and count caps along the top.
Step 3: Calculate the volume of gravel for the leveling base and the drainage stone behind the wall, then convert to cubic yards.
Variables Explained
| Input | Meaning | Unit |
|---|---|---|
| Exposed height | Height visible above grade | ft, in, m |
| Block face length / height | Size of the block’s front face | in, mm |
| Buried course | First course set below grade | yes / no |
| Base trench width, gravel depth | Size of the compacted base | in, mm |
| Drainage stone width | Width of clean stone behind the blocks | in, mm |
Example Calculation
Given: 20 ft long, 3 ft exposed, 16 × 6 in blocks, one buried course, 12 in caps, 24 in wide × 6 in deep base, 12 in of drainage stone
Blocks per course: 240 ÷ 16 = 15
Courses: 36 ÷ 6 = 6, + 1 buried = 7
Wall blocks: 7 × 15 = 105; caps: 240 ÷ 12 = 20
Base gravel: 20 × 2 × 0.5 = 20 ft³ ≈ 0.74 yd³
Drainage stone: 20 × 3.5 × 1 = 70 ft³ ≈ 2.59 yd³
Volume Conversion Table
| Unit | Equivalent |
|---|---|
| 1 yd³ | 27 ft³ ≈ 0.765 m³ |
| 1 m³ | 35.31 ft³ ≈ 1.308 yd³ |
Material Estimation Notes
- Block sizes vary by brand — enter the face size from the product sheet.
- Gravel is sold by volume or by weight; ask your supplier for their conversion, since it depends on the material.
- Compaction reduces loose gravel volume, so suppliers often suggest ordering a little extra.
- Walls with corners, curves or steps need extra blocks and cutting.
Engineering Assumptions
This is a quantity estimate, not a structural design. Taller walls, or walls holding back slopes, driveways or buildings, commonly need geogrid reinforcement and an engineered design. Follow the block manufacturer’s installation guide and local code.
Frequently Asked Questions
How many retaining wall blocks do I need?
Blocks per course × number of courses. A 20 ft wall 3 ft high with 16 × 6 in blocks and a buried course needs 105 blocks.
How much gravel do I need behind a retaining wall?
Wall length × wall height × the width of the drainage zone, converted to cubic yards.
Should the first course be buried?
Many block manufacturers recommend burying part or all of the first course; check your product’s guide.
How deep should the gravel base be?
It depends on the block system, wall height and soil; the manufacturer’s instructions give the recommended base size.
How many cap blocks do I need?
Wall length ÷ cap length, rounded up.