You have decided on the diamond, now you need to figure out her ring size. There are few ways to find out
Get her sister or her friends to go jewellery shopping with her, and ask her friends to try out some jewellery. And in the process try to find out her ring size.
2) Holding Hands
As it is normal for couples to hold hands, she won’t suspect that you are trying to get her ring size. Start by playing with her fingers, and use a visual gauge to find out which of your finger matches closest to her ring finger (fourth finger on the left hand).
With that visual que, measure your finger with the string method to find out her size.
3) Measure current ring
Use this method if she is wearing a ring on her fourth finger, by measuring the diameter of her ring.
4) String Method
a) Get a strip of paper and wrap around forth finger
b) mark the point where the paper meets
c) use ruler to measure the distance between the two points. Do this two to three times to accuracy
d) find out the ring size from the ring size chart