This portion also offers 2 grams fiber, 9 grams of sugar, and 67 calories. Since most bananas we buy are large, it's a good idea to know the carb counts in those as well. One large banana (8 to 9 inches long) will have 31 grams effective (net) carbohydrate, 3.5 grams fiber, 17 grams of sugar, and 121 calories. As you can see, that's double the 1/2-cup serving, which is why it's a good idea for low-carb dieters to stick to the smaller amount.