There are many tables showing the size conversion of a ring from one country’s measurements to the other country’s measurements; but very few publications showing how to measure a finger size correctly for different styles wedding bands.
Many jewelers are measuring the size of wedding bands incorrectly.
In WeddingBands.com we measure the ring size based on the smallest internal diameter of the ring. Wedding bands are made different thickness, width, and profile.
Some 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 ring size should be measured by inside diameter. The smallest inside diameter should specify the size of the ring.
See figure 1 for different made rings should be measured differently on a jeweler mandrel.