Riesling sugar guidelines
The International Riesling Foundation recognises four sweetness categories for Riesling - Dry, Medium Dry, Medium Sweet and Sweet. It strongly encourages Riesling producers to use the same terminology so that consumers are better informed about the different styles.
The designation relies upon the ratio of sugar to acid, and the pH balance. The following table, from the IRF, shows this relationship.
More information can be found at drinkriesling.com/tasteprofile/thescale