On our website (www.QuoVadisPlanners.com) we use both US Customary units (inches) and metric (centimeters) to describe page sizes. This is because we are the US distributor of Exaclair products, the parent company of which is in France.
The measurement system in the United States (USC) was originally developed from English units, which were used in the colonies and England before the Revolutionary War. In England, they later changed to Imperial units (which the US did not adopt) then later to the metric system which is used throughout Europe (and in the US in science and industry). In the UK there are still some holdovers of the previous measuring systems such as people measuring their body weight in “stone” where one stone = 14 lbs, road distances in miles, and of course beer served in pints.
I’m in several planner groups on Facebook where there is much discussion about page sizes for different planners. When giving page dimensions most people understand that US folks are used to measurements in inches and folks outside the US are used to measurements in cm, and adjust accordingly. In the end, most people prefer using the measurement system they are used to.
Do you prefer page sizes measured in inches, or in centimeters? Why?