If you already have a ring that fits you perfectly,
measure the internal diameter and consult the table above:
The ideal ring size is the circumference of your finger.
You can therefore measure the circumference of the chosen finger using a tape measure (take the measurement at the level of the second knuckle, in the middle of the finger).
If you don't have a tape measure at hand, you can use a string that you wrap around your finger (still at the level of the second phalanx).
Make marks on the string at the start and end point.
Then unwind the string and lay it on a ruler.
This will give you the perimeter of your finger in millimeters, which corresponds to the size of the ring.