Very few of us know our ring sizes by heart. Heck. I make rings and I don't even know what size my fingers are!
Below is the most reliable way I have found of measuring ring-size at home.
TIP: Using low-tack tape like masking tape or washi tape will work best.
TIP: When wrapping your finger with the tape, truly go as tightly as you can.
TIP: Be as precise as possible at the marking & measuring steps. Every mm makes a difference!
TIP: It is typically easier to size a ring up than it is to size one down, so if in doubt, err on the slightly smaller size.
Note that if you look up ring size charts online and compare the measurement, you may end up with a different size? Why?
When making rings, we not only have to take into account the size of your finger, but also how thick the material we use is. The above chart is for .8mm thick wire which is about the thinnest you could possibly make a ring out of. Therefore, it is the closest to the thin tape you are using. Othe ring-size charts may be assuming a thicker material...but that isn't very accurate if the ring you are purchasing or having made isn't made out of the same thickness of wire or sheet!
Measuring with a thin piece of tape and comparing to this chart should give you the most accurate measurement based on tests I have done on my own sausage fingers while putting this guide together. X Dee