How Many Retaining Wall Blocks Do I Need? Charts by Wall Size
Divide the wall length by the block width to get blocks per row, multiply by the number of rows, and add one row that sits below ground. For standard 16 × 6 inch blocks, a 20 ft long, 3 ft high wall needs about 111 blocks and 20 caps. The retaining wall calculator does it for any size, along with the gravel.
The retaining wall block formula
blocks per row = wall length (in) ÷ block width (in)
rows = wall height (in) ÷ block height (in) + 1 buried row
blocks = blocks per row × rows, plus 5% for breakage
Use the block's face size: the part you see on the front of the wall. The depth front to back doesn't change the count.
Blocks per square foot
| Face size | Type | Per sq ft of wall face | Per foot of wall, per row |
|---|---|---|---|
| 12 × 4 in | Small garden block | 3.00 | 1.00 |
| 12 × 6 in | Medium block | 2.00 | 1.00 |
| 16 × 6 in | Standard wall block | 1.50 | 0.75 |
| 18 × 6 in | Large wall block | 1.33 | 0.67 |
Blocks and gravel for common walls
Height is above ground. Counts include one buried row and 5% extra. Caps are 12 in long. Gravel is for 16 × 6 in blocks: 6 inches of base in a 24 inch wide trench, and a 12 inch drainage zone behind the wall, both with 15% added for compaction.
| Wall | 12 × 4 in blocks | 16 × 6 in blocks | Caps | Base gravel (tons) | Drainage gravel (tons) |
|---|---|---|---|---|---|
| 10 × 2 ft | 74 | 42 | 10 | 0.6 | 1.5 |
| 20 × 2 ft | 147 | 79 | 20 | 1.2 | 3.0 |
| 20 × 3 ft | 210 | 111 | 20 | 1.2 | 4.2 |
| 30 × 3 ft | 315 | 170 | 30 | 1.8 | 6.3 |
| 40 × 3 ft | 420 | 221 | 40 | 2.4 | 8.3 |
| 50 × 4 ft | 683 | 360 | 50 | 3.0 | 13.4 |
Why you bury the first row
The buried row keeps the bottom of the wall from sliding out under the weight of the soil behind it. Bury at least one full row, or about 1 inch for every 8 inches of wall height if that is more. It's the row people most often forget to count.
Corners, curves and steps
- Corners: 90 degree corners use special corner blocks on many systems. Count the corners and buy one corner block per row at each.
- Curves: measure the face of the wall along the curve. Tight inside curves need a few extra blocks because the backs get trimmed.
- Stepped base: on a slope, the base steps up one block height at a time. Work out each section on its own and add them up.
When you need more than blocks
Most block walls are fine up to about 3 to 4 feet. Taller walls, or walls holding back a driveway, a steep slope or a building, usually need geogrid layers tied into the soil behind, an engineer's design and a permit. Water is what pushes most walls over, so always backfill with clean gravel and add a perforated drain pipe at the bottom.