The measurement is of the inner diameter of the ring.
Please note that the hands are not equal in size, the size of the finger on the right hand is usually not the same as the size of the finger on the left hand.
Try to measure an existing ring that fits the hand you want the new ring on.
Try to measure a ring that is closest to the width of the ring you are purchasing, if there is none, if the ring you are purchasing is particularly wide compared to an existing thin ring, take a little more spare size (wide rings occupy a wider part of the finger and sit tighter on the finger, therefore take it into account and take a slightly larger size in them).
Please note that our fingers expand in the summer when it's hot, and contract in the winter when it's cold. Therefore, it is recommended to measure the existing ring several times, both when it's hot and when it's cold, and make sure that it fits every temperature and doesn't squeeze too much when it's hot and doesn't fall off when it's cold.
Measurement application
The application can be downloaded from the links:
Place a ring you have on the phone screen.
Adjust the size of the circle to the size of your ring.
The size that will appear on the screen is your size.