Yardstickly

Walls guide

How Many Retaining Wall Blocks Do I Need? Chart and Gravel Guide

A 20 × 2 ft wall: 168 blocks, 20 caps and about 2.6 cu yd of stone.

Reviewed by , Editor. Last reviewed . Figures are calculated with the same formulas as our calculators.

A circular paved patio edged with a low stone wall
Low garden walls like this are usually built from small dry-stacked blocks.

Work out the blocks in one row, then the number of rows, counting one buried below ground. A 20 ft wall standing 2 ft high, in 12 × 4 in blocks, needs 168 blocks with 5% spare, 20 caps and about 2.56 cu yd of crushed stone for the base and drainage.

Retaining wall block chart

Blocks 12 in long × 4 in high × 7 in deep, 5% spare, one course buried
WallCoursesBlocksCapsStone
10 ft long, 1 ft high553100.91 cu yd
10 ft long, 2 ft high884101.28 cu yd
20 ft long, 1 ft high5105201.82 cu yd
20 ft long, 2 ft high8168202.56 cu yd
20 ft long, 3 ft high11231203.3 cu yd
40 ft long, 2 ft high8336405.12 cu yd
Metric: blocks 300 mm long × 150 mm high × 250 mm deep
WallCoursesBlocksCapsStone
3 m long, 0.3 m high332100.65 m³
3 m long, 0.6 m high553100.92 m³
6 m long, 0.6 m high5105201.85 m³
6 m long, 0.9 m high7147202.39 m³
10 m long, 0.6 m high5179343.08 m³

Block sizes vary by maker, so measure yours and use the retaining wall calculator.

Worked example: a 20 × 2 ft garden wall

  1. Buried depth: Nicolock's instructions bury 6 in or 10% of the wall's height, whichever is more. 10% of 24 in is 2.4 in, so 6 in.
  2. Total height: 24 + 6 = 30 in. Rows: 30 ÷ 4 = 7.5, so 8 courses.
  3. Blocks per row: 20 ft ÷ 1 ft = 20. Total: 8 × 20 = 160, plus 5%: 168.
  4. Stone: a 6 in levelling pad in a trench 6 in wider than the block at the front and back, and 12 in of drainage stone behind the wall: 2.56 cu yd, about 3.5 tons.

How high can I build?

Nicolock says walls up to about 3 ft can generally be built without reinforcement in good soil. Taller walls, terraced walls and walls on weak soil need geogrid and an engineer's design. Check your block maker's guide and local rules.

Common mistakes

  • Forgetting the buried course. It adds a row of blocks to every wall.
  • No drainage stone. Water pressure behind a wall is a common reason walls lean or fail.
  • Measuring the curve short. Measure curved walls along the front face.

Sources

  1. Nicolock: Retaining wall installation instructions