How Many Retaining Wall Blocks Do I Need?
Block and cap counts for common wall lengths and heights, including the buried course most people forget.
Block retaining walls are bought by the piece, so the count needs to be right. The step people most often miss is the course of blocks that sits below the ground.
The formula
- Blocks per course = wall length in inches ÷ block length, rounded up. A 20 ft wall of 16” blocks needs 240 ÷ 16 = 15 blocks per course.
- Visible courses = visible height ÷ block height. A 24” wall of 6” blocks has 4 visible courses.
- Add the buried course. Most block makers want at least one course below the finished ground (often 10% of the wall’s height, or one block, whichever is more). That makes 5 courses.
- Total = blocks per course × courses, plus about 5% for broken or cut blocks.
- Caps = wall length ÷ cap length, rounded up, plus a spare.
For that 20 ft × 24” wall: 15 × 5 = 75 blocks, plus 5% = 79 blocks and 21 caps.
Block and cap table
Standard 6” × 16” × 12” deep blocks with 12” caps. Heights are the visible wall above the ground.
| Wall length | 12” high | 18” high | 24” high | 36” high |
|---|---|---|---|---|
| 10 ft | 26 + 11 caps | 34 + 11 caps | 42 + 11 caps | 59 + 11 caps |
| 20 ft | 48 + 21 caps | 63 + 21 caps | 79 + 21 caps | 111 + 21 caps |
| 30 ft | 73 + 32 caps | 97 + 32 caps | 121 + 32 caps | 170 + 32 caps |
| 50 ft | 120 + 53 caps | 160 + 53 caps | 200 + 53 caps | 280 + 53 caps |
Corners, curves and steps
- Outside corners often use special corner blocks. Count them separately and take the same number off the regular blocks.
- Curves use roughly the same number of blocks as a straight wall of the same length, measured along the front face. Tight inside curves need cut blocks.
- Stepped walls on sloping ground: work out each level as its own section and add them up.
The rest of the order
Blocks are heavy (standard blocks weigh around 70 lb each), so plan for delivery. You’ll also need:
- Base gravel for a 6” compacted leveling pad.
- Drainage stone, usually clean ¾” gravel, behind the wall.
- A perforated pipe and filter fabric behind the bottom course.
- Concrete adhesive to fix the caps.
See Retaining Wall Drainage for why that stone matters, and How to Build a Block Retaining Wall for the build.
Height limits. Walls approaching 3–4 ft overall often need geogrid, and taller walls or walls holding up a driveway or slope may need a permit and an engineer. Check your block maker’s guide and local rules.
The Retaining Wall Planner counts blocks, caps, gravel, pipe and fabric for your wall and block size.