Ring sizes are not the actual measurement of your finger.  The following information should help you measure to figure out which ring size you should order.  Use a dressmakers measuring tape to measure around the finger you want to use the ring on.  Measure between the first and second knuckle of that finger.  Somewhere above the second knuckle is the optimal position for the ring. 

Ring size converted to circumference of your finger

size 3.5 = 1.79" or 45.40 mm

size 4.5 = 1.80" or 47.97 mm

size 5.5 = 1.99" or 50.58 mm

size 6.5 = 2.09" or 53.16 mm

For example, I wear a size 3.5 and my finger measures just less than 5 cm or 49 mm.  I like the ring to fit snugly on my finger so that it doesn't easily twist around.  I wear the ring on my left middle finger because I knit continental style, and that leaves my left pointer finger to "direct traffic" and hold the left needle.