Green juices contain dark, leafy vegetables like kale and spinach, and are typically chock full of vitamins and minerals. However, most of these beverages also contain fruit juices or purees to balance out the vegetable flavor and to add sweetness. Fruit adds sugar, and is generally more calorie-dense than vegetables. If you're making green juice at home, use more vegetables and less fruit to minimize the calorie content.
The number of calories in your juice will vary by recipe. One grocery-store brand of green juice contains 120 calories per cup, while another major brand's version contains 140 calories in the same serving size. You can make a much lighter beverage at home if you have a juicer. One recipe with apples, kale, lemon, cucumber and ginger contains just 66 calories per cup.