Trying to accurately measure your ring size without an existing ring is quite difficult. Using a piece of string usually ends up pretty inaccurate, you need to check that the size chosen can pass over the knuckles easily without falling off your fingers. If you don’t own a ring, we would strongly suggest borrowing one from a friend that fits, or even finding a cheap ring to measure from.
Using a ring that you already have.
The easiest way to measure your ring size is to use a ring that you already have and that fits comfortably. You also need a ruler with millimeters.
  
1. Place the ring on the ruler. Measure the ring in the widest place.
2. Your ring size is the measure you see on the ruler on the inside of the ring.
Normal sizes for women are about 15.0-21.0 mm and for men men 18.0-24.0 mm.

  
A good tip is to measure your fingers in the morning and evening. Depending on the weather and the time of day, your fingers may vary in size.