Retaining Wall Calculator

Materials for a retaining wall: blocks and caps or sleepers and posts, plus the base and drainage stone, for any length and height.

A segmental block retaining wall is built in courses: the wall height divided by the block height, plus at least one course buried below ground, times the blocks per course. A 30 ft wall 3 ft high in 16 × 6 in blocks has 6 visible courses plus one buried, 23 blocks a course, so 170 blocks and 25 caps with 5% extra. Underneath goes a compacted base, often 24 in wide and 6 in deep (1.6 tons of crushed stone here), and behind it a zone of clean drainage stone with a drain pipe (5.5 tons for a 12 in zone). In Australia, sleeper walls are common: the calculator counts the sleepers, posts and drainage stone for those too.

Units
Base material
Wall
ft
ft
in
in
in
in
in
%
Wall blocks
170

7 courses × 23

Caps
25

one row on top

Base
1.57 tons

Crushed stone / crushed rock, 1.17 yd³

Drainage stone
5.51 tons

4.08 yd³ · plus 30 ft of drain pipe

Taller walls, walls holding back slopes, driveways or buildings, and walls near boundaries usually need an engineer's design and often a permit; many places set the limit at around 1 m (3 to 4 ft). This calculator counts materials only.
How it was worked out
  1. Courses: 3 ft ÷ 6 in block height = 6, rounded up to 6, plus 1 buried = 7
  2. Blocks per course: 30 ft ÷ 16 in = 23; 7 × 23 + 5% = 170 blocks
  3. Caps: 23 + 5% = 25
  4. Base: 30 ft × 24 in wide × 6 in deep of crushed stone / crushed rock + 5% = 1.17 yd³, 1.57 tons
  5. Drainage stone behind the wall: 30 ft × 12 in × 3.5 ft + 5% = 4.08 yd³, 5.51 tons

Common use cases

  • Plan a garden retaining wall or raised bed
  • Level a sloping yard for a patio or lawn
  • Order blocks, caps and gravel together
  • Count sleepers and posts for a sleeper wall

How to use

  1. Choose your units and country (for the base material), and blocks or sleepers.
  2. Enter the wall length and height above ground.
  3. Enter the block or sleeper size, buried courses and the base and drainage widths.
  4. Read the blocks or sleepers, caps or posts, base and drainage stone.

FAQ

How many retaining wall blocks do I need?

Courses = wall height ÷ block height, rounded up, plus the buried course; blocks per course = wall length ÷ block length. Multiply them and add about 5% for cuts. Use the face size printed by the block maker.

What goes behind a retaining wall?

Most block makers specify a zone of clean crushed stone (often about 12 in or 300 mm wide) behind the wall with a perforated drain pipe at the bottom, so water can't build up pressure behind it.

When does a retaining wall need an engineer?

Rules vary, but taller walls (many places set the limit around 1 m, or 3 to 4 ft), walls holding back slopes, driveways or buildings, and walls on boundaries often need an engineer's design and a permit. Check with your local council or building department.

Related tools

Results are estimates for planning and buying materials, worked out from the figures you enter and typical values (weights, coverage, sizes) that vary by product and supplier. They are not engineering, building or professional advice. Check your local building code, the product maker's instructions and your supplier's figures before you buy or build, and use a qualified professional for structural, electrical and gas work. Sizes, spans, depths and reinforcement for anything structural must come from span tables, your building code or an engineer.