Get an Adjustable Plastic Ring Sizer
If you’re worried about any of the other three strategies not guaranteeing accuracy, you can purchase a ring-measurement tool. They aren't too expensive, and you don’t have to leave your house to get this step accomplished.
We have a top-rated Adjustable Plastic Finger Sizer that works like a mini belt, with a sliding plastic arrow marker to measure your ring size ($3). It measures ring sizes 1 through 17 for women, men, and children.