Ring sizes are based on inner diameter. To know what ring size you have you can measure the inside diameter of a ring you have at home with a ruler. A diameter of, for example, 18 mm corresponds to size 18 and so on.
Preferably use a ring whose width / thickness corresponds to the ring you plan to buy. The narrower the ring, the smaller the size can be compared to a wider ring. Also keep in mind that the fingers should not be too cold or too hot and when measuring then it can affect the size. Remember to measure on the finger where you want the ring, our rings look good on different fingers.
Alternatively, take a measuring tape and measure around the finger to get the inner diameter.
You can also visit your local goldsmith or silversmith so they can help you measure your finger.
The most common ring sizes for women are 16.5 mm (S) 17.5 mm (M) 18 mm (L).