How much gravel backfill does a retaining wall need?

A commonly used minimum is a gravel layer at least 30cm (about 12in) thick running the full height of the wall, though your engineer or local guidance may call for more depending on soil type and wall height. This calculator works out the exact volume and weight from your own backfill thickness, wall length, and backfill height.

Can I build a retaining wall without mortar?

Yes — segmental (dry-stacked) concrete retaining blocks are specifically designed to interlock or sit course-on-course without mortar, using their own weight, a slight backward lean (batter), and often a lip or pin system to stay in place. This is the standard method for garden and landscaping retaining walls up to a moderate height; taller or structurally critical walls may use mortared block, poured concrete, or a reinforced/engineered design instead.

Do I need planning permission for a retaining wall in the UK?

Retaining walls are generally treated in the same way as freestanding garden walls under permitted development, but the height thresholds are the same — over 1m next to a highway used by vehicles, or over 2m elsewhere, typically needs planning permission, and any retaining wall that affects the stability of neighbouring land or a boundary may also engage Building Regulations. Check with your local planning authority before building anything more than a low garden edge.

Retaining Wall Calculator — Block Count & Backfill Volume | UK

How many blocks do I need for a retaining wall?

A retaining wall holds back sloped or raised ground so you can level a garden, terrace a bank, or stop a border from eroding onto a path or driveway. Most DIY and small landscaping retaining walls are built from segmental (dry-stacked) concrete blocks, which interlock or sit course-on-course without mortar, backed by a layer of free-draining gravel that stops water pressure building up behind the wall. This calculator works out exactly how many wall blocks and cap blocks you need for a given wall height and length, plus the volume and weight of gravel backfill, and a total estimated material cost.

The maths is a straightforward area problem: your wall is a grid of rows and columns of blocks, so the block count is just rows multiplied by columns, with the top row swapped for flat cap blocks if you want a finished-looking edge. The backfill gravel is a separate calculation — a slab of gravel behind the wall, sized by thickness, wall length, and backfill height, converted to weight using the gravel's density. Both figures matter: under-order blocks and you run short mid-build; skip the gravel backfill and you remove the drainage path that stops a retaining wall failing under water pressure after the first heavy rain.

This guide walks through both calculations with a full worked example you can check by hand, plus the judgement calls the calculator cannot make for you: when a dry-stacked block wall is the right approach, when you need a structural engineer instead, and the drainage details that separate a wall that lasts decades from one that bulges within a few winters.

How to use the retaining wall calculator

Enter the wall height and wall length — the finished face dimensions of the retaining wall you are building.
Enter your block height and block length, either from a manufacturer spec sheet or by selecting your country's preset segmental block sizes.
Choose whether to include a cap row — a row of flat capping units along the top for a finished look and a usable edge — since this changes how the top course is counted.
Enter the backfill thickness (how deep the gravel layer behind the wall is) and backfill height (usually the same as your wall height, or the height you need free-draining material to reach).
Set a waste allowance (5-15% is typical) to cover breakages, cuts, and awkward corners.
Enter a price per block to get a total estimated material cost for the wall blocks, cap blocks, and gravel together.

The retaining wall calculation formula

Block count is a grid calculation: rows = wall height ÷ block height, and columns = wall length ÷ block length, both rounded up since you cannot buy a fraction of a course. Without a cap row, total blocks = rows x columns. With a cap row, the top course becomes dedicated cap units instead of standard wall blocks, so wall blocks = (rows − 1) x columns, and cap blocks = columns (one cap per column along the top).

blocks (no cap) = rows x columns; blocks (with cap) = (rows − 1) x columns, plus columns cap units

Backfill gravel volume is a separate rectangular-slab calculation: volume = backfill thickness x wall length x backfill height. Multiply that volume by the gravel's density to get the weight you need to order, since aggregate is usually sold and delivered by weight (tonne or ton) rather than by volume.

Worked example: a wall 6m long and 1m high, using a 0.4m x 0.2m block, with a cap row included and a 10% waste allowance. Rows = 1 ÷ 0.2 = 5. Columns = 6 ÷ 0.4 = 15. With a cap row: wall blocks = (5 − 1) x 15 = 60, cap blocks = 15, raw total = 75. Add 10% waste: 75 x 1.1 = 82.5, rounded up to 83 blocks in total (68 wall blocks plus 15 caps, after waste is distributed across the order).

Backfill for the same wall, using a 0.3m gravel thickness and a backfill height matching the 1m wall height: volume = 0.3 x 6 x 1 = 1.8 m³. Add 10% waste: 1.8 x 1.1 = 1.98 m³. Using a typical gravel density of 1,346 kg/m³, weight = 1.98 x 1,346 ≈ 2,665 kg (roughly 2.7 tonnes) of drainage gravel to order for this wall.

Pro tips and common mistakes

Drainage is not optional — it is the single biggest factor in whether a retaining wall survives. Water trapped behind a wall with nowhere to go creates hydrostatic pressure dry-stacked block was never designed to resist, and this is the most common cause of retaining wall failure (bulging, leaning, or collapse). The gravel backfill this calculator sizes for you should run the full height of the wall and connect to a drainage pipe at the base, not just sit as a gravel layer with nowhere for the water to exit.

Height is the point where "DIY project" becomes "engineering project." Small segmental block walls (roughly up to 1m/3ft, depending on local rules and the soil being retained) are a realistic weekend DIY project. Taller walls retain disproportionately more soil pressure than the height alone suggests, and most jurisdictions require an engineer-stamped design and a permit above a set height — check your local threshold before committing to a tall dry-stacked wall, since this calculator sizes materials, it does not verify structural adequacy.

Base preparation matters as much as the wall itself. Segmental block walls need a compacted gravel base trench, typically below the frost line in cold climates, so the first course sits level and does not settle unevenly over time — an unlevel first course throws every course above it out of alignment.

Batter (a slight backward lean into the slope) adds real stability. Most segmental block systems are designed with a built-in setback per course so the wall leans back into the hill it is retaining rather than standing perfectly vertical — this uses gravity and the retained soil's own weight to help hold the wall in place, and is not something you should "correct" by stacking blocks perfectly plumb.

Order 5-10% extra blocks even on a simple, straight run. Corners, curves, half-blocks at the ends of a course, and the occasional cracked unit all eat into your raw block count, and matching a specific block colour/texture from a later top-up order is often harder than it sounds.

At a typical UK price of around £4.20 per wall block, the worked example on this page — a 6m long, 1m high retaining wall with a cap row, needing 83 blocks in total including 10% waste — comes out at roughly £348.60 in block cost alone, before gravel backfill, delivery, and labour.

Retaining Wall Calculator