There are 46 calories in 1 cup, diced ( oz) of Watermelon, raw. You'd need to walk 12 minutes to burn 46 calories. Visit CalorieKing to see calorie count and. Find detailed carbs information for Watermelons including Popular Watermelon Serving Sizes and Other Types of Melon. Jun 22, For a quick, low-carb snack, mix cubes of watermelon with onion, fresh tastes great sliced and laid over seared and broiled chicken breasts.