How many blocks for a 20 ft long, 2 ft high retaining wall?
A 20 ft wall showing 2 ft above grade takes 147 standard 12 × 4 in blocks in 7 courses, one of them buried, plus 21 cap units. Under and behind it you need 0.65 cubic yards of base gravel (1.0 tons), 1.90 cubic yards of drainage stone, 24 ft of perforated pipe, and 107 sq ft of fabric. In larger 18 × 8 in blocks the wall takes 59 blocks.
Blocks for a 20 ft × 2 ft wall by block size
| Block | Blocks (5% extra) | Courses | Caps | Block cost, typical |
|---|---|---|---|---|
| 12 × 4 in standard | 147 | 7 | 21 | $412 to $669 |
| 16 × 6 in | 79 | 5 | 16 | $379 to $616 |
| 18 × 8 in large | 59 | 4 | 15 | $425 to $690 |
| 8 × 4 in garden wall | 221 | 7 | 32 | $442 to $718 |
Each count includes one buried course below grade and 5 percent for cuts. Base gravel, drainage stone, pipe, and fabric are the same for every block.
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Base and drainage for a 20 ft wall
- Trench: 10 in deep and about 19 in wide along the full 20 ft, roughly 1.0 cubic yards of digging.
- Base gravel: 0.65 cubic yards of 3/4 in minus, compacted in lifts, about 1.0 tons. See the gravel calculator to price it.
- Drainage stone: 1.90 cubic yards of clean 3/4 in stone for a 12 in column behind the blocks, about 2.9 tons.
- Drain pipe: 24 ft of 4 in perforated pipe with a sock, sloped to daylight at one or both ends.
- Fabric: 107 sq ft of non-woven landscape fabric to separate stone from soil.
- Adhesive: 2 tubes of construction adhesive for the caps and top course.
At this height no geogrid is normally needed, but keep the 12 in of drainage stone and the buried course; they matter more than anything else.
What a 20 ft × 2 ft block wall costs
Materials run about $686 to $1,095: standard blocks at $2.80 to $4.50 each, caps at $3 to $5, gravel and stone at $45 to $65 a ton, plus pipe and fabric. Larger blocks cost more per piece but fewer are needed, so the total lands in a similar range. Installed by a landscaper, segmental block walls typically price at $20 to $40 per square foot of wall face, or $800 to $1,600 for this 40 square foot wall. The blocks alone weigh about 2.2 tons, so plan on delivery.
Common questions
How many retaining wall blocks for 20 feet?
147 standard 12 × 4 in blocks for a wall 2 ft above grade, 79 of the 16 × 6 in size, or 59 of 18 × 8 in blocks, each including a buried course and 5 percent extra.
How much gravel for a 20 ft retaining wall?
0.65 cubic yards for a 6 in compacted base plus 1.90 cubic yards of clean stone behind the wall, about 3.8 tons in total.
Do I need a permit for a 2 ft retaining wall?
Usually not for a wall under 3 to 4 ft that is not holding up a driveway or structure, but check locally. Rules vary by city, so call the building department before you dig.