This plastic ring sizer is the perfect tool to measure your ring size if it is not possible for you to get your fingers sized by a professional jeweler.
Slip ring sizer onto base of desired finger. Tighten (similar to a ziptie) - make sure to tighten it a bit more than it comfortably fits. Slowly remove the ring sizer by pushing it up, paying extra attention as it reaches the knuckle. Make sure that the ring sizer doesn't loosen too much around the knuckle, a tight secure fit that can be removed is ideal. Find your ring size to the left of the arrow indicator on the sizer.
Since the knuckle is the largest part of your finger, make sure that the ring sizer snugly hugs the knuckle as it is removed to get the most accurate ring size. Too loose and the ring would be too big and may be easily lost. When measuring your ring size, keep in mind that your fingers may swell and shrink depending on the time of day. Try taking multiple measurements at different times of the day to ensure accuracy. For pinky rings, try to go down 1/2 a size than the measurement shows since rings on this finger tends to get lost more easily. Again, it is always best to be professionally sized by your local jeweler if possible.