The 4-4-9 Rule
Typically, a gram of carbs and protein is considered to be 4 calories, while a gram of fat is 9 calories. Using these numbers, a food with 15 grams of carbs, 10 grams of protein, and 5 grams of fat should provide 145 calories, calculated as 15 * 4 + 10 * 4 + 5 * 9. For many foods, these "calculated" calories are the same or close to the actual calories printed at the top of the food's Nutrition Facts.
Why Calories Calculated From Macros Differ From Published Calories?
For some foods, though, the "calculated" calories value may be quite different. There are several reasons for that. Most importantly, calories-per-gram for carbs, proteins, and fat (4, 4, and 9 respectively) are approximate, average values, and vary for different kinds of fats, different kinds of carbs, and different proteins. For example, a more accurate "average" calories per gram of fat value is 8.9 calories. For carbs, calories vary from 3.692 calories per gram for glucose to 4.116 calories for starch. Proteins from meat and vegetable sources are metabolized differently, with greater portion lost as nitrogenous substances in urine and feces for meat proteins, thus the caloric effect of one gram of protein is different for them, i.e. it is not precisely 4 calories in all cases.
Thus, the "4-4-9" rule, while working well for many foods, may calculate calories different from calories printed in Nutrition Facts. Differences by a few percent are quite regular. Still, if you see values differing by more than 20%, it is worthy of double-checking Nutrition Facts, to make sure all numbers are correct.
Logged Daily Totals of Calories and Macros Not Matching Planned
These variations of calculated and actual calories may also result in your daily totals of calories and macronutrients not matching planned values.
When calculating your diet plan, total calories are distributed among macronutrients using the "4-4-9" rule, which is considered the most reliable method available. For example, a 2,000 calories plan with 50% of calories coming from carbs, 25% from proteins, and 25% of calories coming from fat means that 1,000 calories should come from carbs, 500 - from proteins, and 500 calories should come from fat. These calories are re-calculated into 250 grams of carbs (that is 1,000 / 4), 125 grams of proteins (that is 500 / 4), and 56 grams of fat (that is 500 / 9)
If all foods had perfect 4-4-9 calories match, then when you consume during a day 250 grams of carbs, 125 grams of proteins, and 56 grams of fat and your total daily calories would be precisely 2,000 calories. BUT, as we now know, calories may not match for some foods, resulting in a discrepancy - you consume carbs, proteins, and fat as planned, but you miss your daily calories! Or, it could be the other way - you hit your calorie targets, but you may miss on some macronutrients!This is a normal situation, caused by one or several foods that have "calculated" calories different from actual, thus causing differences in your planned and actual calories and macronutrients.