Quick Answer
100 grams equals approximately 0.42 cups of water.
For common kitchen ingredients, 100 grams is about 0.8 cups of all-purpose flour, 0.5 cups of granulated sugar, 0.45 cups of packed brown sugar, 0.44 cups of butter, and 0.54 cups of uncooked white rice.
The exact cup amount changes by ingredient because grams measure weight and cups measure volume.