I have always used the rule of thumb of "subtract 10" from the European size to get your American size. For example, a 50R EU becomes a 40R EU,a 54R EU becomes a 44R US, 58R EU becomes a 48R US, and so forth. Most online retailers, SFers and eBayers seem to follow this rule of thumb. The better stores and sellers also provide measurements to give you peace of mind. Far and way, "Subract 10" seems to be the most popular method. I recently became aware that the old school way to convert is this way. I'll quote an explanation from an old SF thread. (Paging @thinman if he still posts here) http://www.styleforum.net/t/296935/what-is-the-origin-of-european-suit-sizing/0_50#post_5375881 I was wondering which method you all personally use when you browse for stuff online? Do you use "subtract 10" or the other method?