An inches scale always comes with equivalent centimeters feature and you can simply count the number of centimeters for given inches.
A 12 inch ruler is about 30 cm long.
Both the centimeter and the inch are used to measure length, but while the former is used in most countries in the world, the inch is used in a couple of countries that still follow the imperial system of measurements, which also includes the foot, yard, mile, etc.
Therefore, when you come back to the website then you don't need to change it again.
Originally the term inch comes from the latin uncia plural: unciae.
There are 36 inches in a yard and 12 inches in a foot.