Retaining Wall Calculator

How the retaining wall calculator works

This calculator finds the volume of a rectangular block of material, such as a concrete wall stem, its footing, or the drainage gravel behind it:

Volume (m³) = length (m) × width (m) × depth (m)

For a wall, "width" is the thickness and "depth" is the height. Work out each part of the wall separately (stem, footing, backfill) and add them up. Enter every dimension in metres, so 200 mm is 0.2.

Worked example: a 10 m concrete retaining wall, 1 m high

1. Wall stem: 10 × 0.2 thick × 1.0 high = 2.0 m³
2. Footing: 10 × 0.6 wide × 0.3 deep = 1.8 m³
3. Concrete total: 3.8 m³, or about 4.1 m³ with an 8% waste allowance
4. Drainage gravel behind the wall: 10 × 0.3 wide × 1.0 high = 3.0 m³

The gravel zone matters as much as the concrete. Water pressure behind a wall is a major cause of failure, so allow for free-draining gravel, a slotted drain pipe at the base and weep holes or an outlet.

When you need an engineer

Retaining walls hold back a lot of force. A metre of wet soil pushes far harder than most people expect. In many Australian council areas, walls over about 1 m high, or walls close to a boundary, a building, a driveway or a slope, need approval and an engineer's design. Rules vary between councils and states, so check before you dig. This calculator is for quantities only. It does not check whether a wall will stand up.

Other wall types

For block walls, calculate the number of blocks from the wall face area and the core-fill concrete from the core volume. Our concrete block calculator helps with that. For timber sleeper walls, divide the wall length by the sleeper length and the height by the sleeper width.

Common mistakes

• Entering millimetres instead of metres.
• Forgetting the footing, which is often as big as the stem.
• Leaving out drainage gravel and ag pipe.
• Measuring the exposed height only. The buried part of the stem counts too.

How much concrete is in a 1 m high retaining wall?

For a 200 mm thick stem, about 0.2 m³ per metre of wall, plus the footing, which is often a further 0.15–0.25 m³ per metre.

Do I need council approval for a retaining wall?

Often, yes, once the wall passes about 1 m or sits near a boundary or structure. Rules vary, so check with your local council before building.

What goes behind a retaining wall?

Typically a 300 mm wide zone of clean gravel, a slotted drain pipe at the base, and geotextile fabric to stop soil clogging the gravel.

Written by Sam Alex, civil engineer. About the author