The Celtic infinity ring is available in various sizes, mostly based on American measurements. To determine the correct size, you can wrap a strip of paper or string tightly around the base of the desired finger, mark the length, and then measure it in millimeters with a ruler. Alternatively, you can measure the inner diameter of an existing, well-fitting ring using a ruler or ring sizer. Since fingers can swell slightly depending on the time of day and temperature, it is recommended to take the measurement several times and, if in doubt, choose a slightly larger size for wider ring models.