There are many tables showing the size conversion of a ring from one country to the other country standard measurements but very few publications showing how to measure a finger size correctly.
Wedding bands are made with different thickness, width, and profile. So it’s quite normal that even professions can make a mistake. According to WeddingBands.com, many jewelers are measuring the size of the wedding bands incorrectly.
The jewelry website that specializes in wedding bands, has released how they measure the ring size, correctly, based on the smallest internal diameter of the ring.
Some wedding bands are made flat inside, some the edges are slightly curved inside and some inside is round where the narrowest part of the ring is in the middle. Normally the size of a wedding band should be measured by the inside diameter of the band . The smallest inside diameter of the band, should specify the size of the ring. Rings manufactured in different profiles, should be measured differently on a jeweler mandrel as shown.