The general rule of thumb is to add 1/2 size and subtract one width from the British EG size to arrive at the American equivalent. Â (In general, the Greens run a bit narrow, so this may be a bit off, but it is reasonably accurate.) Â So, a UK 8E in Green would equate, more or less, to an 8.5D American. Â In my experience, and also based on the original sizing process in the UK and America, in fact the UK lengths (e.g. 8 in example above) are more like 1/3 of a size longer in American sizes (i.e. the equivalent American size would be, if it existed, 8 1/3). Â I am a 8.5/9 American, but I've found the 8 in Green to be perhaps a bit short; I think 8.5 in Green is better for me, though this is still subject to revision. Note that many Greens are marked with both the British and the American sizing. Â The Greens/Purple Labels available on Bluefly ($400 after 20% discount) are listed by American sizes, not British. So, to answer your question, I would think a 11.5 UK
size Green would be great if you are between an 11.5 and 12 American, subject to width issues. Â However, an 11.5 "American" (i.e. 11 UK) might be a bit on the short side, if you are truly between an 11.5 and 12 American. Further, I believe the older Purple Labels were a "D" width and were pretty narrow; more recent versions are a UK E width, which as described below is likely somewhere between a US D and E width. The following is an excerpt of an old post regarding UK/US sizing:
I don't believe that there are, per se, differences in width between the UK and US markings. Â However, the Greens are probably narrower than the given US width would indicate because, I presume, the width (really a girth/all around measurement) of the Green shoes is based on the UK length, and, in shoemaking, width goes up as length goes up -- i.e. a 8.5D is actually Â 1/8 inch bigger around than an 8D. Â So, the result is that a Green 8D is actually 1/2 way between a US Â 8 1/2 C and D "widths" (girth). Â Likewise, a UK 8E is between a US 8 1/2 D and E in width/girth. Â (Note that, also, as width goes up length goes up -- but (at least in theory) only in the part of the shoe in front of the ball -- so a UK 8E is actually a bit longer than a UK 8D.) Â Therefore, a Green 8E (UK size) is a bit wider/bigger girth than a US 8.5D and about the same length as the 8.5D, but with slightly different proportions; the heel to ball measurement is shorter than that of a US 8.5D. For what it's worth, it is also an anomaly of shoe-fitting that most customers think the "widths" are a measure of the width of the insole across the ball of the foot; this is incorrect. Â As noted above, he "width" is actually a measure of the girth (circumference) of the last; therefore, even two shoes with the same "width" measurement can have different insole widths, as the shape of the lasts may differ and one may have more of the total girth/circumference in the top part of the foot -- manufacturers often like to keep the ball width narrow because a.) they can use the same insoles for multiples sizes, and b.) they think it makes the shoe and foot look slimmer and more elegant. Â Furthermore, typically the "length" measure is from the heel to the ball of the foot; different lasts can have very different measures from the ball of the foot forward, based both on the manufacturer's preferences and the shape of the last/shoe. Â Finally, shoemakers don't necessarily stick to the purported standards, and there is of course deviation around the mean, so their shoes can vary even more. Â All of this says that it's important to actually try shoes on before buying them, particularly if you're not familiar with the manufacturer's sizing and last shapes.