If you want to order a ring as a surprise for someone and you need to find out their size, the best way is to borrow a ring from them that you know fits them well (and specifically fits the finger you want them to wear your gift on) and bring it to any jewelry shop for measuring.
The other methods are less accurate, but I will still name a couple that helped my customers over the years.
- If you already have a ring that fits you (or the person you’re buying the ring for) well then you can put it on top of a ruler (important - measure in millimeters, not inches) and see how many millimeters you have between the inner sides of the ring. The diameter in millimeters will be your size. For that purpose the ring needs to be regular round shape and not too chunky, otherwise the calculations won’t be too precise.