Calories per Gram of Protein, Carbs and Fat

Protein and carbohydrate each carry 4 calories per gram; fat carries 9. Those three numbers are the conversion factors behind every macro calculation, and in the United States they are written into food-labeling regulation rather than into custom.

The numbers, and where the law puts them

Component	Calories per gram
Protein	4
Carbohydrate	4
Fat	9
Alcohol	about 7

The first three are the general Atwater factors. Federal regulation, at 21 CFR 101.9(c)(1)(i), lets a manufacturer compute the calorie declaration by “using the general factors of 4, 4, and 9 calories per gram for protein, total carbohydrate, and total fat, respectively, as described in USDA Handbook No. 74.” They are named for Wilbur Atwater, whose late-1800s work measured the energy in food and subtracted what the body fails to absorb or excretes unused — which is why protein comes to 4 rather than the 5.6 a bomb calorimeter reads.

Alcohol is the odd entry, and its 7 comes from the same Atwater series rather than from the regulation quoted above — federal labeling rules give alcohol no macro line at all. Its energy is folded into the total calorie count without being attributable to protein, carbohydrate or fat, which is why a drink can make the three macros sum to less than the calories shown.

Converting in both directions

Grams to calories — multiply and add. A plate with 40 g of protein, 60 g of carbohydrate and 20 g of fat carries (40 × 4) + (60 × 4) + (20 × 9) = 160 + 240 + 180 = 580 calories.

Calories to grams — take the percentage, then divide by the factor. Thirty percent of 2,000 calories is 600; as protein that is 600 ÷ 4 = 150 g, as fat it is 600 ÷ 9 = 67 g. Same calories, less than half the weight of food. That asymmetry is the practical content of the number 9, and the reason oil, nuts and cheese add up so quickly while looking like small additions.

Why the label sometimes disagrees with your math

Add the macros on a package, multiply by 4/4/9, and you will often land 5 to 20 calories away from the printed total. Nothing is wrong. Three mechanisms explain the gap:

• Rounding. Fat is declared to the nearest half gram below 5 g and the nearest gram above, and calories to the nearest 5 or 10. The declared numbers are already approximations of each other.
• Specific factors. The same regulation allows specific Atwater factors for particular foods — the energy in the protein of eggs differs from the protein of beans — so a manufacturer using them will not reproduce the general 4/4/9 result.
• Fiber and sugar alcohols. Non-digestible carbohydrate may be counted at 2 calories per gram, and each sugar alcohol has its own value in the rules: xylitol 2.4, maltitol 2.1, sorbitol 2.6, erythritol 0.

The last one matters for anyone tracking a low-carbohydrate diet, where sugar-alcohol products are common and their calorie count is not the 4 you would assume.

Using the factors

Once the conversion is second nature, the rest of macro planning is division. Start from your calorie total, choose a split, and the macro calculator does the arithmetic above in both directions. For the goal-specific versions, see macros for weight loss and macros for muscle gain; for the individual macros, the daily carb calculator and the daily fat intake calculator.

This content is informational and does not replace advice from a physician or registered dietitian.