Find a ring that fits you correctly. Consult the Cartier Ring Size Chart.
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.
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.