Theoretically, the height of man is often referenced as the scale comparison devise by envisoning a six-foot man alongside the object being scaled designating the man equal to an inch makes the 20-foot high wall equal to 3.33 inches
Now it would be great if we could leave it that way but...
Although the metric system has specified a limited series of scales for blueprints and maps, when it comes to models, there may be a problem with these scales for a readily imagined person of 180 centimeters. Model railways have the additional difficulty of having to present the rail gauge as a simple number, the height of a person being secondary.
Trade authorities in metric countries are attempting to specify scales that are simple multiples of 2 and 5, but neither tracks nor people seem to fit. In such cases, rationalization may actually be invoked for competitive advantage, to prevent interoperability with products from another manufacturing country.
On the other hand, wargaming scales have traditionally been traced to metric system, where the number of millimeters relate to the relative height of the human figure based on 180 cm standard man.
Therefore 25 mm scale (popular in historical and fantasy wargaming) refers to 1:72 scale, whilst the 15 mm scale (nowadays the most popular scale in ancient, medieval and Renaissance wargaming) refers to 1:120 scale (Many manufacturers refer to 15 mm as 1:100 scale). Likewise, 50 mm scale is the same as 1:35 military model scale, and 5 mm equals 1:350 naval scale.