It helps to get it right when giving someone a ring as a gift.
Though if you do get it wrong, you are in good company. When Queen Elizabeth II was proposed to by Prince Albert the ring was too big!
See my article on keeping it a secret for some ideas on how to find out the correct size.
If you want to surprise someone with a ring you might have to check out what diameter any of their current rings are. Once you have the measurement you can check the size on the International Ring Size Chart below.
There are four different conventions used in regard to ring sizes. Just make sure the website or jeweler you buy from is using the same convention that you are using. Some sites detect what country you are in when you bring up their website on your computer. They may automatically list all the ring sizes on the website according to your country’s sizes, but they do tell you, so don’t worry too much about this.
If you have access to a ring that belongs to the person for whom you are buying a ring, you can determine its size by laying the ring on a ruler and making note of the inside measurement of the ring. This gives you the diameter.
If you don’t have a ruler nearby, but do have a photocopier or scanner, you can get a copy of the ring and measure it later. If you don’t have a ruler laying around, and don’t have access to a photocopier or scanner but do have some plasticine, blue-tac, clay or even a slice of bread you can also get an impression of the ring. Make sure whatever you use does not leave any residue in the ring. Then you can measure the diameter later when you can find a ruler.
Here is the International Ring Size Chart which you can use to find out the correct size ring to buy.