Macro Calories Calculator

Kcal per gram: the Atwater factors

Behind every food label and macro calculator sit the Atwater factors, which Wilbur Atwater published around 1900: protein = 4 kcal/g, carbohydrate = 4 kcal/g, fat = 9 kcal/g, alcohol = 7 kcal/g. The formula is just kcal = 4P + 4C + 9F + 7A. Example: 100 g protein + 250 g carbs + 60 g fat = 400 + 1000 + 540 = 1,940 kcal. Where do the numbers come from? Bomb calorimetry (food burned in pure oxygen), then discounted for what gets lost to digestion. Full combustion releases more energy than the body ever pulls out. Fiber is its own case: soluble fiber lands around 2 kcal/g, since gut bacteria ferment it into short-chain fatty acids, while insoluble fiber sits near 0 kcal/g. As for trans fats, Brazil banned them through ANVISA Resolution RDC 332/2019 (effective 2023), so they are no longer produced industrially.

Applications: label reading, calorie counting, custom diets

These factors are what produce the "Energy" line on nutrition labels, which is computed straight from the macros. They run inside calorie counting apps (MyFitnessPal, Cronometer, FatSecret), they help nutritionists build custom diets, and the food industry R&D teams lean on them when reformulating a product as "light" or "low-calorie". They also make a handy sanity check. Say a product claims 200 kcal but its macros add up to 280 kcal — the label is wrong, or rounded with a heavy hand. With alcoholic drinks, keep that 7 kcal/g in mind: a 5% ABV beer of 350 mL carries roughly 14 g of alcohol, about 100 kcal from the ethanol alone.

FAQ

Why does fat have more than double the calories of protein? Gram for gram, fat molecules pack more carbon-hydrogen bonds, and breaking those bonds releases more energy. Carbs and proteins already carry oxygen in their structure, which drags down how much they yield when burned.

Are Atwater factors exact? Not really; they are averages. Almonds are the classic example: they actually deliver around 4.6 kcal/g rather than the 6 kcal/g their macros predict, because part of the fat passes through undigested. For everyday tracking, that gap is small enough to ignore.

Does fiber count as a carb? On Brazilian labels it gets folded into "total carbohydrates". In keto circles people subtract it out to land on "net carbs". Neither view is wrong; they just answer different questions.

What about polyols (erythritol, xylitol)? They fall somewhere between 0 and 2.4 kcal/g. Erythritol comes in near 0 kcal/g because the body excretes it intact, while xylitol runs about 2.4 kcal/g. Labels usually break them out separately under "sugar alcohols".