Topsoil Calculator NZ
This calculator works out how much topsoil or garden mix to order for a lawn, garden bed or landscaping job, converting the area you are covering into both a volume and a weight so you can order confidently, whichever unit your supplier quotes in. You enter the area in square metres, the finished depth you want in millimetres, and the density in tonnes per cubic metre, which defaults to 1.3 for standard topsoil but can be adjusted lower for a lighter garden mix or higher for a heavier clay-based soil. The calculator instantly returns the volume needed in cubic metres and the equivalent weight in tonnes, updating as you change any figure. For example, a 25 square metre lawn topped up 100 millimetres deep needs 2.5 cubic metres of topsoil, or about 3.25 tonnes at the default density. Use it before ordering bulk soil so you are not caught short partway through a job, and remember that soil settles once spread and watered in, so order a little extra for a level finish. If your supplier has given you a density figure, enter that instead of the default for a more accurate result. This is a planning estimate based on the figures you enter; actual quantities can vary with compaction and moisture content, so treat the result as a guide rather than an exact order quantity.
Soil settles, so order a little extra for a level finish.
How it works
The volume is the area times the depth in metres. Multiply by the density to get the weight. Topsoil is around 1.3 tonnes per cubic metre, while light garden and compost mixes weigh less. Use the figure your supplier gives if you have it.
Worked example
For a 25 square metre lawn at 100 mm deep, the volume is 2.5 cubic metres. At 1.3 tonnes per cubic metre that is about 3.25 tonnes of topsoil.
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