Measure your finger for a ring with an official ring sizer or use a piece of string to calculate the circumference you need.Jewellers use an official size guide for rings, but each country has its own labelling system.In this guide we’ll explain how to measure your ring size, as well as how to find out someone's ring size without asking them – so you can surprise them with the perfect fit. Whether you’re looking for an engagement ring, wedding ring, or simply a gift for yourself or someone else, you need to know how to find out the correct ring size. Rings can be resized but you don’t want to be on one knee, sliding the ring on the finger and for it to stick half way. When it comes to buying a ring, it’s important to get the perfect fit.