Food has water content, and water has zero calories.
When you do the math on the components, fat, protein and carbs, it tends to come close to the ultimate value.
http://www.umich.edu/~elements//web_mod/potato/fact.htm
Seems these folks have the potato as 80% water and 20% solids.
Let's see how close we get using the values on the web page:
Potato = 148g
Carbs = 26g
Fat = 0g
Protein = 4g
30g is about 20% of the 148g total, so that part is consistent with the measurement of fat, protein and carbs.
Calories = 4*c+4*p+9*f or 4*26 + 4*4 +0 = 104+16 or about 120 calories.
The carbs and protein must be rounded or some of the carbs are considered non-digestible (fiber.) I cannot explain the difference between the 120 calculated and the 100 depicted on the web page linked above. I suspect some error is allowed,
USDA claims a 213g potato is 163 calories. That's about .765 calories/gram of potato. The umich website above has our 148g potato with 100 calories, or about .676 calories/gram.
Dunno who has it right.
Originally Posted by sleepery
I am a bit confused when looking at caloric content of food weighed in ounces vs. grams .
The site calorie king lists 5.3 oz of potato at 116 calories .
5.3 ounces converted to grams = 150 grams .
At 4 calories per gram roughly, that would be 600 calories ? I am not sure if I am making a obvious mistake .