If you are finding out the size for yourself, I can take your size in person or you can go to any reputable jewelry store.
I recommend whenever possible using a set of thin ring sizers and a set of wide ring sizers to get a more accurate size. The wider the inside of the ring - especially if the inside is flat - the bigger the size will have to be to go comfortably over the knuckle.
The ring finger on the hand you use to write with will be a bit bigger than the other hand so it’s a good idea to size them both. If you like to wear rings in other fingers, you might as well get those sized as well.
Weight fluctuations, pregnancy, very salty meals and even temperature changes can affect your ring size. Everyone’s knuckles get bigger as we age so that many people need to have their rings resized as they get older. If you ring is loose once it goes over the knuckle tightly, there are options to adjust for that.
The most important thing to keep in mind is temperature. Your ring size can vary between 1/4 to 1/2 size depending on wether your hands are cold or hot. So I recommend making sure that the ring size is tight enough that if your hands are warm you will have to struggle a little bit to take the ring off. You can always cool down your hands or use a little windex or soap to get a ring of but you do not want the ring to come off easily when your hands are cold as it increases the risk of loosing it, especially when taking gloves off.
Ring sizers are usually a collection or rings attached to a larger hoop to keep them in order. They are come in 1/2 size intervals. If you find that you are between sizes, you can fine tune the size.
For example if you are between a size 5 and 6 and 5 is tight but six is a bit too loose, try 5.5.
If 5.5 is a bit tight but not too much and 6 is a bit loose, then you are a 5 3/4.
If 5.5 is tight and 6 is better but comes with not much of a struggle, then you are 5 7/8.
Go through the process with a thin ring sizing set and a wide ring sizing set so you have your size range.
You can also take along a ring that first perfectly and have that sized. Be careful though because if the ring is no longer perfectly round, the reading might be wrong. That’s why it’s important to go to a jeweler that knows how to properly size a ring.
Also keep in mind that it is usually easier to make a ring bigger by up to a 1/4 size or so than it is to make it smaller. To make a ring bigger, I can stetch the metal. To make it smaller, I need to remove metal.
Rings with very delicate stones or stones evenly spaced are more difficult to size and in the case of eternity rings, they cannot be sized. In some cases laser welding is an option, another option is to get a properly sized ring to wear next to the larger ring so it stays in place.