How many?

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.

Kilometres

The kilometre, or kilometer in American English, is a unit of length in the metric system, equal to one thousand metres. Many people know a marathon race is 26.2 miles long but did you know that a marathon is 42.195 in kilometres?