Need to get your ring size. I suggest to get the most accurate measurement is to go to the nearest jewelry store and use their professional ring sizer. However, if you're like me and live 20 mins away where one is not very convenient then this should do the trick. I'm also one of these that I want to know right now...especially if it's in the middle of night. ;)
I've come across this printable ring sizer that seems to be pretty accurate when used correctly. It matches up to my ring mandrel anyway. Simply print this image out at a 100% and cut it out along the black line also cute the center line that is vertical next to the letter "A". (Don't worry about the 3 1/2 inches, you don't use that for the ring sizer part.) Once cut out, slide the pointed end through the cut slit by the letter A, so the print is facing outward. Then place the finger you want to measure inside and pull the tab. You have to pull as snug as possible to get the most accurate reading I've found. When I mean snug I mean pull the tab to the point you feel as though your going to rip the paper. I do on my finger in order to get the right measurement. If there is white space above the number it means you are the previous number and 1/2. Example if a 6 shows with white space on top you are most likely a 5.5 ring size.
Also, I place tape on top of and around the area of where I cut the slit to help prevent it from ripping. I hope this helps.