You can't. If it is outside, you can go directly to the jewelry store and ask the staff to measure it for you. They have professional measuring tools, so they will be more accurate. Knowing your finger size, you can find the ring number that suits you on the comparison table according to the obtained data. Although there are many versions of the ring size comparison table, what remains unchanged is your finger circumference, so you only need to remember this data and look it up in the corresponding size comparison table, generally there will be no big deviation. If your finger data is between two ring numbers, it is recommended to choose a larger number; In addition, when measuring, try to avoid high temperature environment and severe cold. Fingers will expand with temperature and contract with cold. Therefore, for the accuracy of the data, it is generally recommended that you choose the number of rings that feel tighter in summer and slightly looser in winter.