The metric unit commonly used for measuring the length of small objects is the millimetre (mm).

In the metric system, the millimetre is defined as one-thousandth of a metre, represented mathematically as $1 \text{ mm} = \frac{1}{1000} \text{ m}$. This unit is particularly advantageous for measuring small objects with a high degree of precision. For instance, when measuring the thickness of a piece of paper, the diameter of a small screw, or the length of an insect, millimetres offer a convenient and accurate scale for measurement.

To provide further context, it’s important to note that there are $10$ millimetres in a centimetre and $1,000$ millimetres in a metre. This hierarchical structure of the metric system facilitates easy conversions between different units. For example, if you measure an object to be $25$ millimetres long, you can quickly convert that measurement to $2.5$ centimetres by dividing by $10$.

When using a ruler or measuring tape, millimetres are typically marked as the smallest divisions. This feature allows for precise measurements, which is crucial in fields such as engineering, science, and even in everyday tasks like DIY projects. A solid understanding of millimetres can enhance your accuracy and efficiency in making measurements.

In summary, the millimetre serves as the primary metric unit for measuring small objects, valued for its precision and ease of use within the broader metric system.