When purchasing rings it is important to get your fingers accurately sized by a professional jeweler.
The sizing must be recent as finger size can vary with time depending on a variety of factors. Intense heat or cold can change your finger size as can exercising, dieting, medication, travel (especially air travel), swimming and other activities. We recommend you get your fingers sized when things are relatively normal. Just don't go after you have been to a sauna.
Please do not use webtools such as paper cutouts or string for finger sizing. They do not work! These methods are notoriously inaccurate. To avoid any surprises please make sure you select or provide a ring size from a recent fitting with a professional jeweler.
A professional jeweler will use metal ring gauges to size your fingers. There are specific gauges for narrow and wide band rings. Typically wide band rings require a slightly larger size to fit properly. Stacking rings have similar requirements. Each hand is slightly different so please make sure you size the finger on the hand you intend to wear your rings.
There is nothing more disappointing than ordering a custom-made ring and finding it does not fit. Take the time to measure your fingers properly at a professional jeweler - one who uses metal ring gauges of small incremental sizes - and all should be well.
And if you are planning a surprise - say an engagement ring - don't try to measure your partner's finger with a piece of string while they are asleep. They always wake up! And the size you get will not be accurate. Instead, borrow a ring of theirs that you know fits. A professional jeweler will be able to size it for you. You can always pretend they lost it if they notice - and then have the double pleasure of finding it for them again. Or if they don't have a ring you can use try and get friends or family to take them 'shopping' and find it out for you.