On the USDA database the entry for [this chicken thigh](https://fdc.nal.usda.gov/fdc-app.html#/food-details/1899680/nutrients), calculating the sum of its parts using the 4-9-4 rule (4 cal per gram of carb/protein and 9 cal per gram of fat) is ~20 cal off from it’s listed energy amount. A similar noticeable difference can be found on [this nutrition label for some edamame](https://www.instacart.com/store/items/item_1910923160).
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Why does this happen? I know there are tools like bomb calorimeters. Do those provide different numbers than the 4-9-4 rule? If so, is that what they use to list the total calories on the label?
In: 4
The 20 kcal is probably rounding. Protein carbs and fat are usually rounded to the nearest gram, and total kcal are rounded to the nearest 5 or 10.
Also not all carbohydrates are 4cal/gram. Fiber is counted as a carbohydrate, but depending on the type, it might actually have 0 or 2 kcal/g.
The rules set by the government for counting and calculating this stuff aren’t all clear or consistent. In general, you should treat nutrition labels as rough estimations, not precise measurements.
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