Retaining Wall Block Calculator NZ

This calculator works out how many blocks you need to build a retaining wall in New Zealand, from the wall length and height and the face size of your chosen block, plus a small allowance for cuts and breakages. Whether you are using keystone-style interlocking blocks, besser blocks or concrete units, the principle is the same: the number of blocks is set by how much wall face you have to cover and how big each block's face is. Working it out before you order saves both the cost of over-buying heavy, awkward blocks and the delay of running short partway up the wall. The maths is simply the wall face area, length times height, divided by the face area of one block, and rounded up. You enter the wall length and height in metres and the block face length and height in millimetres, plus a waste allowance, and the calculator returns the number of blocks needed along with the wall face area. As a guide, many retaining blocks have a face around three hundred by two hundred millimetres, but they vary a lot, so use the real dimensions from your supplier's spec sheet. Importantly, retaining walls over about 1.5 metres or carrying extra load usually need a building consent and engineering in New Zealand, so check with your council before you build. An estimate for the block quantity only.

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105
retaining wall blocks needed
Wall face area6.0 m²
Per block face0.060 m²

Use your block's real face size from the spec sheet. Walls over about 1.5 m or carrying load usually need consent and engineering. An estimate for quantity only.

How it works

The wall face area is the wall length times the height. The face area of one block is its length times height, converted from millimetres to square metres. The number of blocks is the wall area divided by the block face, increased by the waste allowance and rounded up.

Worked example

A 10 metre wall, 0.6 metres high, is 6 square metres of face. With 300 by 200 millimetre blocks, each covers 0.06 square metres, so 100 blocks plus a 5 percent allowance is about 105 blocks.

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Frequently asked questions

How many blocks do I need for a retaining wall?

Work out the wall face area, length times height, and divide by the face area of one block, length times height of the block. Round up, and add a few percent for cuts and breakages. This gives the number of blocks for the visible face.

What block size should I use?

Common retaining blocks have a face of around 300 by 200 millimetres, but they vary widely by product, so use the actual dimensions of the block you are buying. The supplier's spec sheet gives the face size and coverage.

Do I need consent for a retaining wall?

In New Zealand, retaining walls over a certain height, often 1.5 metres, or walls carrying extra load, usually need a building consent and engineering. Check with your council before you build. This tool only estimates block quantity.