1. If you have the opportunity go directly to your trusted goldsmith to figure out exactly what size of all your fingers are, in case this is not possible, you will need to use a perfectly round band ring in the size you know is perfect for your chosen finger.
2. Once you have it, place the ring on a ruler to take the measurements. Let us start from the correct position, as you can see in the photo, the ring should be "cut in two" symmetrically from the line on the ruler, in this way we are going to measure the part that is most convex and protrudes, thus obtaining the correct measurement. Calibrate the "0" so it corresponds with the beginning of the inside measurement.
3. The measurement we will get from point A to point B corresponds to the inner diameter of the ring, so we will be able to trace our size by counting in millimeters, in this example 15mm=SIZE 7, so the correct size to order is size 7.