You've heard of these scales forever; 6mm, 15mm, 20mm, 28mm etc. Well, to be truthful, these are not scales at all. They are measures of distance. A scale is "1/4" or "1/72nd" or "1/100th", these are ratios between the dimensions of the real item and the model. For example, if I am making a model of a house in 1/285th scale, and the front door is 79" tall I simply divide the door's height by 285 and I get a scale height of .277", pretty simple really. What if I want to make the same house in 15mm?, how about 20mm? What do I divide or multiply by? You see the problem? 15mm and 20mm are not scales.
So where did these wargaming "scales" come from? And why do vehicle and structure modelers hate them so much?
Wargaming scales expressed in millimeters are actually the measurement of a "standard" male figure measured from the bottom of his boots to the middle of his eyes. So a 15mm figure should measure 15mm from the bottom of his feet to his eyes.
There are many problems with this; How tall is a "standard male figure"? Are we talking about Roman era men? WWII era Germans or WWII era Asians? Modern North American men? Modern European men? Nobody agrees on any of this. Don't even get me started on elves, gnomes and hobbits. So, a lot of figure makers use the "that looks right" rule when making their figures.
Then there is the phenomenon known as "scale creep". This is where someone makes a figure that is just barely larger than another figure he or she is comparing it to, but it looks "close enough". Then the figure maker makes another figure sometime later and compares it to latest work he or she did and even though about a millimeter taller, it still looks good. The cycle repeats over and over and over until someday we end up with a 15mm figure that more like a 20mm figure.
This causes problems for people that make vehicles and structures for wargaming.
Personally, I have had people write me in the past telling me things like "the doorway on this 15mm building is not tall enough, if my 15mm figure tried to walk through it he'd hit his head for sure." Then I ask the person to measure their figure from the bottom of the guy's feet to his eyes and he comes back with a dimension like 17 or 18mm (already over sized), then add to that the 1.5mm - 2mm thick base that is part of the figure, and we are up to about 20mm or 21mm from the bottom of the base to the top of the helmet. Lets not even mention the 3mm thick Flames of War base he's glued the figure which takes us up to about 23mm or 24mm. Of course the figure looks silly next to 72" tall doorway that is 18.3mm tall in this scale .... but the doorway is not the problem. However, it's hard to convince someone that the building is not wrong; the figure is wrong. The popular thinking is to make the building "look right", but to a model maker those are fighting words, we don't want it to look right, we want it to BE right and many of us take pride in making it right ... down to the thousandth of an inch.
So, even assuming that figures are correctly sized, how does a model maker making vehicles or structures get around the millimeter based non-scales? We come up with scale equivalents. Since there is no such thing as a standard male figure we have to make some assumptions. Based on those assumptions most model makers have come to the consensus that;
3mm = 1/600th scale
6mm = 1/285th scale (some say 1/300th scale)
10mm = 1/160th scale
12mm = 1/144th scale
15mm = 1/100th scale (some say 1/110th scale)
20mm = 1/72nd scale (some say 1/76th scale)
28mm = 1/56th scale
With these conversion factors, model makers can accurately make vehicles and structures in wargaming scales. We can only wish that the figure makers used these same scales instead of the "that looks about right" rule.