How many?

Millimetres

The millimetre or millimeter is a unit of length in the metric system, equal to one thousandth of a metre. There are one thousand millimetres in a metre. There are ten millimetres in a centimetre. The millimetre is used for measuring thinner things. For example, wooden flooring and underlay is often measured millimetres

Micrometres

The micrometre or micrometer, also commonly known as a micron, is an SI derived unit of length equalling 1×10−6 metre; that is, one millionth of a metre (or one thousandth of a millimetre, 0.001 mm, or about 0.000039 inch). The micrometre is often used to measure the thickness or diameter of microscopic objects, such as microorganisms and colloidal particles.