The most accurate way to measure your finger is to have a jewler with a ring sizer measure your finger. Most jewlery stores will do this as a free serivice. If you are unable to access a jewler, you may use this printable ring sizer.
1.Print out the sizer on this link
2.Cut out the ring sizer. Make a slice where indicated.
3.Wrap the ring sizer around the largest part of your finger. This is usually your knuckle.
4.Insert the end of the tab into the slot.
5.Pull the tab until the sizer is as snug as you can get it without ripping the paper.
6.The arrow should point to your ring size.