Imperial and Metric
Before designing your own PCB, you need to know the standard measurement units. Most of you may wonder why imperial units (e.g., inches) shall be used as measurement units when designing PCBs, that’s because the majority of electronic components were and still are manufactured with imperial pin spacing. If you learn metric well, that’s ok. But you’d better start learning about inches and how to convert them, because metric will make the laying out of your board a lot harder, messier and may even make it more expensive to produce.
An old saying for PCB design is thou shall use thous. A thou is 1/1000th of an inch, and is universally used and recognized by PCB designers and manufacturers everywhere. In addition, the term mil is as important as the thou. It comes from 1 thou being equal to 1 milli-inch, and 1 mil is the same as 1 thou.
As a general rule, avoid using mil and stick to thou. But in the practical world, you have to use both imperial inches (thous) and the metric millimeter (mm). So you have to remember that thous are used for tracks, pads, spacings and grids, and only use mm for hole sizes and board dimensions.