The metric unit used for measuring large volumes is the cubic metre, denoted as $m^3$.

In the metric system, the cubic metre serves as the standard unit for volume measurement, particularly when handling substantial quantities. A cubic metre is defined as the volume of a cube with edges measuring one metre in length. This unit is especially beneficial in fields such as construction, shipping, and large-scale liquid storage, where smaller units like litres would be impractical due to the vast sizes involved.

To illustrate the scale of this measurement, one cubic metre is equivalent to $1,000$ litres. This means that a container with a volume of one cubic metre can hold $1,000$ litres of water. This conversion is particularly useful, as it allows for easy switching between units based on the context. For instance, while the volume of a swimming pool is often expressed in cubic metres, the volume of a water bottle is typically measured in litres.

In addition to cubic metres, other units such as cubic centimetres ($cm^3$) and cubic decimetres ($dm^3$) are also employed, though these are more suitable for smaller volumes. Notably, one cubic decimetre is equivalent to one litre, and one cubic centimetre is equivalent to one millilitre. Understanding these relationships is essential for converting between different volume units, which is a vital skill across various real-world applications, ranging from cooking to engineering.

Therefore, when it comes to measuring large volumes, the cubic metre is the preferred unit in the metric system.