How Many Calories are in 100 grams and 1 cup of Grapefruit or Grapefruit Juice. 100 grams of grapefruit has 32 calories and it includes 3% fat (0.1g), 90% carbs (8.08g), 7% protein (0.63g). 1 cup of grapefruit has 74 calories and it includes 3% fat (0.23g), 90% carbs (0.23g), 7% protein (1.45g).