How many concrete blocks per m² for a retaining wall?

Standard concrete blocks (440 x 215mm face) require approximately 10 blocks per m² when laid with 10mm mortar joints. This is the same calculation used for any blockwork retaining wall. For a wall 5m long x 1m high (5 m²), you would need 50 blocks plus 10% wastage = 55 blocks. This retaining wall material calculator covers blocks, sleepers and gabions.

Do retaining walls need foundations?

Yes — all retaining walls need a concrete foundation (footing). The foundation should be at least 150mm deep and wider than the wall. For concrete block walls, the foundation should be at least 300mm wide. Pour the concrete into a trench dug below topsoil level on firm, undisturbed ground. The Type 1 sub-base can be used under sleeper walls — see our Type 1 Sub-Base Calculator (/landscaping/type-1-sub-base-calculator).

How high can a retaining wall be without planning permission?

In England and Wales, retaining walls up to 1 metre high adjacent to a highway, or 2 metres high elsewhere, generally do not require planning permission under permitted development rights. However, walls over 1.2m high should be designed by a structural engineer. Building Regulations may still apply. For fencing instead, see our Fence Post Spacing Calculator (/landscaping/fence-post-spacing-calculator).

Do retaining walls need drainage?

Yes — drainage behind a retaining wall is critical to prevent water pressure building up and pushing the wall over. Install a French drain along the back of the wall — use our French Drain Calculator (/landscaping/french-drain-calculator) for gravel and pipe quantities. Weep holes at the base of blockwork walls also help relieve water pressure. Calculate the pipe gradient with our Drainage Fall Calculator (/landscaping/drainage-fall-calculator).

What is a gabion retaining wall?

Gabion walls use wire mesh baskets filled with stone or aggregate. They are excellent for retaining walls as they are flexible, free-draining and very strong. Standard baskets are 1m x 0.5m x 1m (L x H x D). They do not need mortar and can be stacked to create walls up to 3-4 metres high with proper engineering. Fill the raised bed behind with soil — use our Raised Bed Soil Calculator (/landscaping/raised-bed-soil-calculator) or Topsoil Calculator (/landscaping/topsoil-calculator).

Retaining Wall Calculator — Blocks, Sleepers & Gabion Baskets UK

Retaining Wall Calculator — Blocks, Sleepers & Gabions

Estimate blocks, railway sleepers or gabion baskets needed for a retaining wall, plus foundation concrete volume.

Wall Length

Total length of the retaining wall

Wall Height

Height of the retaining wall

Wall Material

Include wastage allowance

Adds 10% extra for cuts and breakages

Wastage Percentage

10% recommended for most materials

Price Per Block (optional)

Enter your supplier price for a cost estimate

How We Calculate This

Our retaining wall calculator estimates the number of blocks, sleepers or gabion baskets needed based on your wall dimensions, plus the concrete required for the foundation.

Material calculations

Concrete blocks (440 × 215 × 100mm): 10 per m² of wall face
Railway sleepers (200 × 100 × 2400mm): Courses = height ÷ 0.2m, per course = length ÷ 2.4m
Gabion baskets (1000 × 500 × 1000mm): Baskets high = height ÷ 0.5m, per row = length ÷ 1.0m

Foundation calculation

Foundation volume = Wall length × Foundation width × 150mm depth

Concrete density: 2.4 tonnes per m³. Foundation width depends on the wall material (typically 300mm for blockwork and sleepers).

Important safety notes

Walls over 1.2m high should be designed by a structural engineer
All retaining walls must have adequate drainage behind them
Planning permission may be required depending on location and height
Building Regulations may apply for walls retaining significant loads

Frequently Asked Questions

Last updated: February 2026

All calculations are estimates. Verify with your supplier.

Retaining Wall Calculator — Blocks, Sleepers & Gabions

How We Calculate This

Material calculations

Foundation calculation

Important safety notes

Frequently Asked Questions

Related Calculators

Topsoil Calculator

Gravel Calculator

Type 1 Sub-Base Calculator

Soakaway Sizing Calculator

Fence Post Spacing Calculator