If you are planning to give a platinum ring as a surprise, here are a few tips to help find the recipient’s ring size:
Ask a friend or family member that you can trust with a secret if they know her ring size.
If she wears a ring on her engagement finger, wait until she removes it and get an impression of it by imprinting it in soap or clay and take it to the jeweler.
Draw around the inside of the ring. Your jeweler can take an accurate measurement from this.
Push the ring as far down on your finger as possible and mark it with a pen; again the jeweler can take a measurement from this.