Unfortunately, the 7 1/8 size is a diameter measurement from the forehead to the Frypan target at the back of the head, while Akubra's Metric measurements are a circumference. So to translate to an approximate figure, we multiply the diameter by Pi to get a circumferential figure, then multiply by 2.54 to convert to cm.
7.125 x 3.14159 x 2.54 = 56.85cm
Of course, heads aren't round, so this is not a perfect conversion. If this is all too hard to work out in that egg-shaped head, go to
this conversion website!