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Generated on: July 20, 2026 at 09:33 PM

Retaining Wall Blocks Calculator

Inputs

Wall length6 m
Wall height0.9 m
Block length45 cm
Block height20 cm
Waste allowance5 %

This is an estimate for a straight wall built from uniform rectangular blocks. Actual quantities depend on the block system, cap blocks, corners, curves, and the buried base course, and tall or load-bearing walls may need engineering, geogrid reinforcement, or a permit. Follow the block manufacturer’s installation guide and local code.

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Construction & DIY

Retaining Wall Blocks Calculator

Estimate how many segmental retaining-wall blocks a wall needs from its length and height and the size of one block, including a waste allowance for cuts and breakage.

Inputs

β‰₯ 0 m
β‰₯ 0 m
β‰₯ 0.1 cm
β‰₯ 0.1 cm
%
0 – 50 %

Results

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Details

A 6 m Γ— 0.9 m wall takes about ... blocks β€” ... courses of ... blocks each, plus waste.

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Retaining Wall Blocks

A segmental retaining wall is built from interlocking concrete blocks stacked in courses, and the number of blocks follows directly from the wall face β€” its length and height β€” and the size of one block. This calculator counts the courses, the blocks in each course, and the total with a waste allowance.

Counting courses and blocks

The number of stacked rows is the wall height divided by the block height, rounded up because a partial course still needs a full row of blocks:

C=⌈HbHβŒ‰\begin{aligned} C &= \left\lceil \frac{H}{b_H} \right\rceil \end{aligned}

The blocks in one row come from the wall length divided by the block length, again rounded up:

n=⌈LbLβŒ‰\begin{aligned} n &= \left\lceil \frac{L}{b_L} \right\rceil \end{aligned}

Multiplying the two and adding a waste fraction kk for end cuts and breakage gives the total to order:

N=⌈C n (1+k)βŒ‰\begin{aligned} N &= \left\lceil C \, n \, (1 + k) \right\rceil \end{aligned}

Worked example

Consider a wall 6 m long and 0.9 m high, built from blocks 0.45 m long and 0.2 m high, with a 5% waste allowance:

C=⌈0.90.2βŒ‰=⌈4.5βŒ‰=5n=⌈60.45βŒ‰=⌈13.3βŒ‰=14N=⌈5Γ—14Γ—1.05βŒ‰=⌈73.5βŒ‰=74\begin{aligned} C &= \left\lceil \frac{0.9}{0.2} \right\rceil = \lceil 4.5 \rceil = 5 \\ n &= \left\lceil \frac{6}{0.45} \right\rceil = \lceil 13.3 \rceil = 14 \\ N &= \left\lceil 5 \times 14 \times 1.05 \right\rceil = \lceil 73.5 \rceil = 74 \end{aligned}

So the wall takes about 74 blocks.

Consider a wall 20 ft long and 3 ft high, built from blocks 18 in long and 8 in high, with a 5% waste allowance:

C=⌈368βŒ‰=⌈4.5βŒ‰=5n=⌈24018βŒ‰=⌈13.3βŒ‰=14N=⌈5Γ—14Γ—1.05βŒ‰=⌈73.5βŒ‰=74\begin{aligned} C &= \left\lceil \frac{36}{8} \right\rceil = \lceil 4.5 \rceil = 5 \\ n &= \left\lceil \frac{240}{18} \right\rceil = \lceil 13.3 \rceil = 14 \\ N &= \left\lceil 5 \times 14 \times 1.05 \right\rceil = \lceil 73.5 \rceil = 74 \end{aligned}

So the wall takes about 74 blocks.

Choosing inputs

Use the nominal block dimensions from the manufacturer, since the listed size usually includes the small setback or lip that lets courses interlock. The buried base course is included in the count because it uses the same blocks. Straight walls with square ends waste little, so 5% is usually enough; curves, steps, and corners need cut blocks and favour 10%.

For the compacted gravel base beneath the wall, see Gravel Calculator.

Frequently Asked Questions (FAQ)

How many retaining wall blocks do I need?

Divide the wall height by the block height to get the number of courses, rounding up, then divide the wall length by the block length to get the blocks in each course, again rounding up. Multiply the two and add a waste allowance. A wall 6 m long and 0.9 m high built from 45 Γ— 20 cm blocks needs 5 courses of 14 blocks, about 74 blocks with 5% waste.

How much waste should I add for wall blocks?

About 5% covers a straight wall with square ends. Curved walls, stepped walls on a slope, and walls with corners or returns waste more because blocks have to be cut or split to fit, so 10% is safer for those. Blocks from different pallets can vary slightly in colour, so ordering a small surplus from one batch avoids a mismatch on a later repair.

Does the base course count?

Yes. The bottom course is usually set below grade on a compacted gravel base, and it is included in the course count here because it uses the same blocks. The buried base course is what keeps the wall from sliding, so it is laid carefully and levelled before the courses above go up.

Disclaimer

This is an estimate for a straight wall built from uniform rectangular blocks. Actual quantities depend on the block system, cap blocks, corners, curves, and the buried base course, and tall or load-bearing walls may need engineering, geogrid reinforcement, or a permit. Follow the block manufacturer’s installation guide and local code.

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