Measure your finger at the end of the day. Do not measure your finger when it is hot or cold.
Remember that the two hands differ slightly in size; a ring worn on a finger of the left hand will not be a perfect fit for the same finger on the right hand. Make sure that the chosen size leaves enough room for the ring to pass easily along the entire length of the finger.
To measure your finger, you can use a tape measure. Measure the circumference of your finger in mm. This measurement corresponds to the size of the ring.
If you are hesitating between two sizes, choose the larger size. Please note that the style of the ring will also affect the choice of size: a large ring is made to be worn more loosely than a smaller model.