The measurement "tonnes" is actually a measurement of weight. The measurement "cubic metres" is a measurement of volume. Thus, for different materials of different density – that is a greater weight by volume – the conversion factor will be different.

In the context of landscaping, one cannot put a definite conversion rate, since there are many different substances that may be used by landscaping supplies stores and plant nurseries. There are different densities of soil, and there are different densities of materials used with soil, such as mulch.

The simplest conversion is water – one cubic meter of water weighs one tonne – all other conversion factors are less than or greater than this, based on the density of the material in question. For example, 1 cubic metre of soil will weigh approximately 1.5 tonnes, because soil is denser than water. This will vary for different soils. It will also vary for different levels of compaction. If the soil is packed, you may want to bump it up to 2.0 tonnes to the cubic metre. If the soil is looser, you may want to reduce it to 1.25 tonnes per cubic metre. Generally you can use a similar conversion for sand as well.

Conversion between cubic metres and tonnes can be tricky. If you are at all unsure, ask experts at your local nursery or contact a landscaping architect.