If you are unsure of the ring size you want to order, you can follow these tips to find your size:
Tips for Measuring:
1. You can purchase a ring sizer (it's inexpensive and always useful), or if not, use a printable ring Sizer: There are printable ring sizers available online that you can use by printing them out and following the instructions to measure your finger.
2. Use a string or thread: If you don’t have paper, you can also use a piece of string or thread. Simply wrap it around the base of your finger and mark where the ends meet. Then measure the length of the string with a ruler.
3. Measure multiple times: For accuracy, measure your finger at least two or three times to ensure you get a consistent measurement.
As a result, you should know the circumference measurement of your finger and the diameter of that circumference:

Sizing ref.
USA Sizing ref.
(Remember to mention in custom orders):
4.5/ 47.8 mm circumference
5/ 49.2 mm circumference
5.5/ 50.5mm circumference
6/ 51.9mm circumference
6.5/ 53.1mm circumference
7/ 54.4mm circumference
7.5/ 55.7mm circumference
8/ 57.0mm circumference
8.5/ 58.3 mm circumference
9/ 59.5mm circumference
9.5/ 60.8mm circumference
10/ 62.1mm circumference