Millimetre Wikipedia open wikipedia design. The millimetre is a metric unit of length. The symbol for the millimetre is mm. There are 1,000 millimetres in a metre. This short article about mathematics can be made longer. You can help Wikipedia by adding to it. This page is based on a Wikipedia article written by contributors (read/edit).Text is available under the CC BY-SA 4.0 license; additional terms may apply. Images, videos and audio are available under their respective licenses.