How Many Blocks for a 20 ft Retaining Wall, 3 ft High?

A 20 ft long wall standing 3 ft above the ground needs 111 blocks in the standard 16 × 6 in size, counting one buried course and 5% extra, plus 20 caps and about 5.4 tons of gravel for the base and drainage. At typical US prices the materials cost about $811–$1,533.

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How it's worked out

  1. Blocks per course: 20 ft × 12 = 240 in ÷ 16 in = 15 (rounded up)
  2. Courses: 3 ft × 12 = 36 in ÷ 6 in = 6, plus 1 buried course = 7
  3. Blocks: 15 × 7 = 105, plus 5% for breakage = 111
  4. Caps: 240 in ÷ 12 in caps = 20
  5. Base gravel: 24 in wide × 6 in deep trench, +15% for compaction = 1.2 tons
  6. Drainage gravel: 12 in behind the wall for its full 3.5 ft, +15% = 4.2 tons

20x3 wall with other block sizes

Block faceTypeBlocksBlock priceMaterials cost
12 × 4 inSmall garden block210$2–$3.5 each$723–$1,362
12 × 6 inMedium block147$3–$5 each$753–$1,380
16 × 6 inStandard wall block111$4.5–$8 each$811–$1,533
18 × 6 inLarge wall block103$6–$12 each$930–$1,881

Materials cost covers blocks, 20 caps at $3–$7 each, base and drainage gravel at $40–$75 a ton in bulk, and 20 ft of 4 in perforated drain pipe. It leaves out tax, delivery, landscape fabric and tool rental. Ask for a pallet price if you need this many blocks; it's often cheaper than buying them one at a time.

At 3 ft high this wall is close to the limit for an unreinforced block wall. Above about 3 to 4 ft, or if it holds up a driveway or a steep slope, it usually needs geogrid, an engineer's design and a permit, so check with your town first.

Other common sizes

More detail: How Many Retaining Wall Blocks Do I Need? Charts by Wall Size