This is a sizing guide, and not a replacement for sizing by a jewelry professional.  Each ring will fit slightly differently due to the width of the shank, and under-gallery configuration.  

THINGS THAT AFFECT RING SIZE
• Water intake
• Salt and salty food (including: Margaritas, and Sushi)
• Weather:
    - Warm weather causes fingers to swell
    - Cold weather causes fingers to contract
• Medications
• Physical Activity:
    - Running
    - Working out