Buy the wrench according to the size of the FLATS on the pedal spindle, not the thread size. I use a 15mm which is the size of the FLATS on the pedal spindle, all of my bikes are this size. Do you have pedals with different size flats, which is the way I read the original post? If you have a pedal which has 1/2" FLATS, you'll need 13mm.