Tear up a strip of paper (book pages are vetoed, you know how much we love them)
Wrap the paper snug around your finger, make sure it's nice and tight and mark the spot where it meets
Measure the distance with a ruler (in mm); that's your finger's circumference
Fingers tend to swell throughout the day, so it's better to measure in the evening.
Beware that thicker rings feel tighter than thinner ones. You may want to choose a size up for thicker bands.
Who likes cold hands? Not us, since the cold can shrink your fingers (and therefore your hard-earned measurements)