There are approximate conversions for recipes, since many American recipes use volume measures and expect you to have measuring cups, while European recipes expect you to have a kitchen scale. But yes, there isn't any single conversion, since density varies: there's one cups/grams ratio for granulated sugar, one for powdered sugar, one for sifted flour, one for water, etc.
Which, as a geek, I find superconfusing and generally insane. I wish cooking was treated as chemistry (which it de facto is) and at least used precise units and proper measuring tools.
