Ordering the right ring size when you’re buying online can be a bit tricky. This is partly due to the different tools used by different jewelers, and also because different parts of the world use different ring measuring systems (there are a number of charts available online that make it relatively easy to convert from one system to another, though).
To make sure you order the right size, I strongly recommend that you visit a local jeweler (or two or three!) to have your ring size professionally measured before ordering. This should be free of charge, & really is the only way that you’ll get an accurate size (using a strip of paper or a piece of string unfortunately isn’t going to work all that well!).
Please note that the size you need will depend not just on which finger you want to wear your ring on, but also on the width of the ring band – rings with wider bands need to be a bit bigger than ones with narrow bands to fit comfortably. Most of my rings have fairly narrow bands (2 – 4mm), though, so this shouldn’t be an issue for most orders.
If you’re ordering a ring as a gift & don’t want to ruin the surprise, you can either order a ring with an adjustable band or you can get an estimate of their ring size by measuring the internal diameter of a ring they already wear. If you’re going to give this a go, please use a flat ruler (not a measuring tape) and take the measurement in millmetres, being careful to take the internal measurement (i.e. from the inside of the ring on one side to the inside of the ring on the opposite side) at the widest point of the ring. This is not an exact method, though, so please let me know if you’re doing this & we can work together to ensure the best fit.
And one last thing, please remember that the size of your fingers (or those of the person you’re giving a ring to as a gift!) will change with changes in season / temperature, weight, etc.