width of the K and F are similar around the middle part of your foot and in terms of instep. However K is much slimmer in the toe box vs the F lastThe K is like the U in fit: They come to a point and squeeze toes if you're not careful in sizing. Don't believe this stuff about K being wider than F. That's a joke. F fits me great with a little extra room at size 45. K at 45 strangled my toes, but I could have worn them with discomfort. Like others have mentioned, K and U width improves if you move a half size bigger, but then they would be a little too long.