How many blocks do I need for a retaining wall?

1. The formula, in plain English

A segmental retaining wall is built like a brick of steps turned on its side: you stack whole courses (rows) of blocks from a level base up to the height you want, and each course is a whole number of blocks laid end to end. That makes the count a grid problem with two rounding rules. The first is the number of courses — divide the wall height by the height of one block and round up, because you cannot leave a course half-built. The second is the blocks in each course — divide the wall length by the width of one block and round up, because the end of the wall takes a full or cut block, never a gap. Multiply the two numbers and you have the bare block count.

Both roundings matter more than they look. A wall that comes out at 6.1 courses still needs 7 full rows, and a run that works out to 20.2 blocks per course still needs 21. That is why a quick “area divided by block face” estimate under-orders: it ignores the fact that you buy whole blocks in whole rows. The retaining wall calculator applies both round-ups for whatever block size you set, then adds your waste allowance on top.

2. A worked example

Say you are building a garden retaining wall 20 ft long and 3 ft high from a standard 12 × 6 in block. The courses are 3 ft ÷ 6 in = 6 rows. The blocks per course are 20 ft ÷ 12 in = 20 blocks. So the bare count is 6 × 20 = 120 blocks. Add a 5% waste allowance for cuts and the odd cracked unit and you are ordering about 126. If you are finishing the top with a capping row, that is one cap per block-width of length — another 20 caps on a 20 ft wall. Underneath, a base 12 in wide and 6 in deep along the 20 ft run is about 0.37 cubic yards of compacted gravel.

The metric version works the same way. A wall 6 m long and 1 m high in 300 × 150 mm blocks is 1 m ÷ 150 mm = 7 courses (the height rounds up from 6.7), and 6 m ÷ 300 mm = 20 blocks per course, so 7 × 20 = 140 blocks, or about 147 with 5% waste. Notice how the height rounding quietly adds a whole extra row of 20 blocks — proof that it pays to count courses properly rather than scale a rough area figure.

3. Block-count chart for common walls

Here are the bare block counts (before waste and caps) for common wall sizes, using a standard 12 × 6 in block — so each foot of length is one block and each course is 6 in of height. For a 300 × 150 mm metric block the same counts apply to a wall measured in the equivalent metres.

A bigger block changes everything: switch to an 18 × 8 in unit and the same 20 × 3 ft wall drops from 6 courses of 20 to about 5 courses of 14 — roughly 70 blocks instead of 120. Larger blocks lay faster with fewer joints, but each cut wastes more, so a curved or stepped wall can justify a higher waste percentage. Always count in the block you will actually buy.

4. The base, the caps and the backfill

The blocks are only part of the order. Under the bottom course goes a levelling base of compacted gravel that spreads the load and gives you a dead-flat first row — get this wrong and the whole wall follows. Its volume is just length × base width × base depth, the same volume maths the gravel calculator uses; a base around 12 in (300 mm) wide and 6 in (150 mm) deep is a common starting point for a low wall, but check your block maker's spec because taller walls need more. On top, a capping row (often a dedicated cap unit glued down) finishes the wall — count one cap per block-width along the length.

Behind the wall you will also want free-draining gravel backfill and usually a perforated drain pipe, so water does not build up and push the wall over. That backfill volume depends on how far back you fill, but it is the same length × width × depth calculation. None of this is structural design — it is the standard kit of parts for a drained block wall — so treat the quantities as planning figures and follow the manufacturer's installation guide for the specifics.

5. Height, engineering and permits

Counting blocks is the easy part; keeping the wall standing is not, and that is a structural and regulatory question, not a calculator one. As a general guide, many jurisdictions let a homeowner build a low landscape wall — often up to about 1.2 m (4 ft) measured from the bottom of the base — without an engineered design. Above that height, or whenever the wall retains a slope, a driveway, a pool or any structure, you typically need a permit, an engineer's design and geogrid soil reinforcement tied back into the bank. Surcharge loads (a parking area or building near the top) lower that threshold further.

We deliberately do not print a single “legal” height here, because the limit, the reinforcement rules and the permit triggers are set by your local building authority and vary by place and soil. Use this guide and the calculator to size the materials, then confirm the height limit and reinforcement before you start. If you are pouring a concrete footing or a stepped foundation for the wall, the concrete calculator works out that volume separately.

6. Buying blocks: batches, spares and ordering

Retaining-wall blocks are sold by the unit, the layer or the pallet, and like brick and pavers the colour can vary subtly between production batches. Order the whole job — including the waste margin and the caps — in one delivery so the wall is built from a consistent batch, and keep a few spares stored dry for future repairs. Round up to whole blocks (and whole pallets where the supplier sells that way), since you cannot buy part of a unit, and double-check the block's face size your supplier actually stocks against the size you entered, because “standard” covers a range of close-but-different blocks.

A retaining wall is one piece of a wider hardscape budget. Laying a patio or path on the level ground it creates? The mulch calculator and the other estimating tools on the landscaping hub use the same measure-once, add-waste approach. Work each material out before the delivery turns up and you will order close to right the first time.

Common questions

How do you calculate how many blocks you need for a retaining wall?

Find the number of courses (rows) by dividing the wall height by the block height and rounding up; find the blocks per course by dividing the wall length by the block width and rounding up; then multiply the two and add 5–10% for waste. A 20 ft long, 3 ft high wall in 12 × 6 in blocks is 6 courses of 20 blocks = 120, or about 126 with a 5% allowance.

How many blocks do I need for a 20 foot retaining wall?

It depends on the height and block size. For a 20 ft wall built from 12 × 6 in blocks, each course is 20 blocks: a 2 ft wall is 4 courses (80 blocks), a 3 ft wall is 6 courses (120), and a 4 ft wall is 8 courses (160), before adding a waste allowance and a capping row.

How much gravel do I need for the base?

The compacted-gravel levelling base is a trench under the bottom course: its volume is length × base width × base depth. A base about 12 in (300 mm) wide and 6 in (150 mm) deep is a common starting point for a low garden wall — roughly 0.37 cubic yards on a 20 ft run — but confirm against the block maker's instructions, because taller or load-bearing walls need a wider, deeper base.

How tall can a retaining wall be without engineering?

Many areas allow a low landscape wall — often up to about 1.2 m (4 ft) — without an engineered design, but above that, or where the wall retains a slope, a driveway or a structure, a permit, geogrid reinforcement and an engineer's design are commonly required. The exact limit is set locally, so check with your building authority before you build.