For most people, measuring buttons is a case of just giving a measurement or picking a design you like. In production, we measure the size of a button using something called “Ligne”
Wikipedia describes the history of the word as:
In the 9th century German button makers began to use the term ligne to measure the diameter of buttons. The consensus definition was that a ligne was the measurement of a round wick, folded flat. In this sense it measures 1⁄40 of an inch, but not exactly, for there were several inches in the kingdoms and petty states of Germany at that time.
Such a measurement became the American measurement called “line,” being one-fortieth of the US-customary inch, used measure buttons, probably introduced by German immigrants. It remains in US use today for buttons and snaps.
Ligne is actually measured differently in different countries, but the most widely accepted is the English version, which says that 1 inch is equal to 40 ligne or 25.5 mm. [ In France however, 1 inch is equal to 11 ligne ]. This is mainly why most books and online sites will suggest to divide the measurement of the button in inches, by 40, which is not very accurate.
An accepted way to work out ligne is : Button Diameter in millimetres / 0.635 = Button Ligne
But rather than having to work it out, here is something to help you…..