Calorie Count. You’ll get around 130 calories from one king crab leg that weighs 134 grams, or about 5 ounces. Nearly 80 percent of the calories, or 105 calories, are from protein. King crab legs do not contain carbohydrates, so the remaining 20 percent of calories, or about 25 calories, are from fat. You’ll get nearly 6.5 times your recommendation of vitamin B-12 from a single king crab leg.