I notice on the inside of the tongues of my running shoes the size of the shoe in USA, UK, and Europe. Of course Europe uses mm., but both we and the UK use inches; however, UK sizes are always one inch smaller. Why?
IIRC, the US convert from Childrens sizes to Adult sizes a size earlier than UK charts. The UK childs sizes go up to 15, while the us sizes stop at the equivalent of 14.
I wasn’t aware that UK shoe sizes were based on inches. I believe the Americans use inches and we use some weird, sui generis, arbitrary system, devised buy some deranged medieval cobbler.
The european sizes are not cm (I assume you ment cm rather than mm) either. I just measured my feet and they are NOT 43 cm long - that would be about 17 inches!!!
I’m a size 6 1/2 American and size 4 UK, so if both systems really are based on inches (doubtful) the difference is much greater than one inch.
From a little travel program in my Visor:
US:__4___4.5___5___5.5___6.5___7.5___8___9___9.5___10
UK:__3.5_4.5___5___5.5___6_____7_____8___9___9.5___10
EU:__36__37____38__39____40____41____42__43__44____45
It makes absolutely NO SENSE whatsoever. Well, as always, the European ones seem to make sense 
Well, my size 11 (USA) is a size 10 in the UK. Go figure. Does anyone have an explanation?
Twistoff, I didn’t mean to ignore your answer, which makes sense and would solve the inch difference, but Coldfire put cold water on that. Well, he got off something in his visor, and hat makers are not shoe makers. What do they know? I guess I’ll have to accept your answer until something better comes along. Still, the cm in Europe doesn’t correspond to inches. Another problem.
The US sizing system is only very loosely based on inches. Payless ShoeSource has a chart with which you can determine your own shoe size, based on foot length.
Looking at it, one sees that an increase in foot length of one inch corresponds to a three-size change - this is true for both men’s and women’s sizes[sup]*[/sup]. Each [sup]1[/sup]/[sub]2[/sub] size, the smallest increment generally used, corresponds to [sup]1[/sup]/[sub]6[/sub] inch length change. They round those numbers to the nearest sixteenth of an inch, so none of the differences actually show up to be a sixth, but if you study the whole pattern you can see what I mean.
The numbers themselves are arbitrary. Extrapolating, a women’s size 0 would be a 7-inch foot, and anything shorter than that would be a negative size.
[sup]*[/sup]Actually, the men’s sizes on their chart seem to start deviating from this in the very small sizes.
From my little Collin’s Gem Ready Reference guide:
Men’s shoes
UK USA Europe
6.5 7 39
7 7.5 40
7.5 8 41
8 8.5 42
8.5 9 43
9 9.5 43
9.5 10 44
10 10.5 44
10.5 11 45
Women’s shoes
UK USA Europe
3.5 5 36
4.5 6 37
5.5 7 38
6.5 8 39
7.5 9 40
I also have figures for children’s shoes, and male, female and children’s clothes, but I’m not posting them all unless you really want them.