We always measure the width on the inside of the buckle! Not the outside of the buckle. It is basically the width of the leather strap that goes into the buckle.
|Title||Oval centre bar buckle solid brass 12,5 mm (gold)|
We measure the inside of the buckles, O-ring etc. (= the size of the leather belt that can go through it)
The size of O-rings is the inside diameter, and with the D-rings we measure the inside of the straight side.