Retaining Wall Calculator NZ
This calculator works out how many concrete blocks you need to build a retaining wall, so you can price the job and order the right quantity before you start. You enter the wall length and height in metres, plus the length and height of the block you plan to use in millimetres, matching the size sold by your local supplier. From these figures it returns the total number of blocks needed, the blocks per row, and the number of rows required to reach your wall height, so you can see how the total is built up. Each row and block count is rounded up to the next whole block, since you cannot buy a fraction of one, and the figure does not allow for corners, curves or steps, which typically need a few extra blocks. Retaining walls need more than blocks alone. Budget for a compacted hardfill base, drainage metal and a perforated pipe behind the wall to relieve water pressure, and for taller walls, geogrid reinforcement or posts. In New Zealand a building consent is generally required once a wall passes 1.5 metres in height, or for lower walls carrying extra load such as a driveway, so check with your council before you build. This is a materials estimate only, so confirm quantities with your supplier and get engineering advice for anything beyond a straightforward low wall.
Blocks only. Add base course, drainage metal and pipe, and get advice for walls over 1.5 m.
How it works
Blocks per row is the wall length divided by the block length. The number of rows is the wall height divided by the block height. Multiplying the two gives the total, rounded up to whole blocks. Curved or stepped walls and corners may need a few more.
Worked example
An 8 metre wall, 0.9 metres high, with 295 by 150 mm blocks needs about 28 blocks per row and 6 rows, which is around 168 blocks.
Related calculators
- Concrete Calculator: concrete for footings.
- Gravel Calculator: drainage metal.
- Brick Calculator: bricks for a wall.
- Fencing Calculator: posts and palings.