If you have a ring that fits today, measure it, it is a safe way!
If you have access to a caliper and someone who can read it or if you can you will get a very good answer to which ring you should buy.

You measure the diameter in mm. straight across the inside, see picture.
Can also go with a fine ruler if you are careful!

You can also use this method.

Different width - Different size

The size of a ring varies depending on how wide the ring is and what the rail looks like.
That's why you have different ring sizes to measure your finger with.

A rule of thumb is: +0.25 size (diameter) for every extra 3 mm width of the ring.

For example, you have a ring today that is 4 mm wide and it is 21 mm in diameter.
But you want a new one that is 10 mm wide.
Then it is 6 mm wider than the one you have, which is + 0.5 mm.
You should then buy one that is 20.5 mm.

Visit a jewelry store.

If you ask, they will certainly help with this.

It is not strange to ask for such a service
and they have the "tools".