Do you have a good food scale? If you want to be accurate, you'll need to weigh it and figure out how many ounces or grams it weighs. According to the official USDA food list on our site
, 100 grams of sweet potatoe contains 86 calories. That's around 3 1/2 ounces. They also state that a 5 inch sweet potato might weigh around 130 grams (given as an example), though that can vary.