The energy value of foods is determined primarily through calculation, not direct measurement. Food manufacturers and government databases use a system of standard conversion factors: 4 calories per gram for protein, 4 calories per gram for carbohydrate, and 9 calories per gram for fat. These numbers, known as the Atwater factors, have been the foundation of calorie counting for over a century. But the full picture involves several layers of science, from burning food in a sealed chamber to accounting for what your body actually absorbs.
Burning Food in a Bomb Calorimeter
The most direct way to measure a food’s energy is bomb calorimetry. A dried, weighed food sample is placed inside a sealed steel chamber, pressurized with pure oxygen, and ignited. The chamber sits in a water bath, and as the food burns completely, the rise in water temperature reveals exactly how much total chemical energy the food contained. This value is called gross energy.
Gross energy represents every last calorie locked in a food’s chemical bonds. The problem is that your body isn’t a furnace. You don’t extract 100% of that energy. Some passes through undigested in feces, some is lost in urine (particularly from incomplete protein metabolism), and a portion fuels the digestive process itself. So while bomb calorimetry gives a useful starting point, the number on a nutrition label needs to reflect what your body can actually use.
How Atwater Factors Work
In the 1890s, the American chemist Wilbur Atwater built one of the first human respiration calorimeters, a room-sized instrument where subjects could live for one or more days while researchers precisely tracked all the energy going in and coming out. By measuring the food people ate, the heat their bodies produced, and the energy lost in urine and feces, Atwater confirmed that the law of conservation of energy applies to human metabolism just as it does to any physical system.
From this work came the general Atwater factors still used today: 4 calories per gram of protein, 4 calories per gram of carbohydrate, 9 calories per gram of fat, and 7 calories per gram of alcohol. These aren’t the raw combustion values from a bomb calorimeter. They already account for average losses during digestion and metabolism, giving an estimate of “metabolizable energy,” the portion your body can put to work.
To calculate calories, a lab first analyzes a food’s composition: how many grams of fat, protein, and carbohydrate it contains per serving. Carbohydrate is typically calculated “by difference,” meaning analysts measure water, protein, fat, and ash (mineral content), then subtract all of those from 100. Whatever remains is classified as carbohydrate. The gram amounts are then multiplied by the corresponding Atwater factors and added together. That sum is the calorie count you see on the label.
General Factors vs. Specific Factors
The 4-4-9 values are general averages, and they treat all proteins, fats, and carbohydrates as interchangeable. In reality, the fat in coconut oil has a slightly different combustion energy than the fat in olive oil, and the protein in eggs is more digestible than the protein in legumes. To address this, researchers Merrill and Watt published updated tables in 1955 (revised in 1973) with “specific Atwater factors” tailored to individual food groups.
The difference can be meaningful. For nuts, the specific factors for protein, fat, and carbohydrate are 3.47, 8.37, and 4.07 calories per gram, compared to the general 4, 9, and 4. For almonds specifically, that distinction shifts the calorie estimate noticeably. The USDA nutrient database uses these specific factors when available, while most commercial food labels rely on the simpler general factors.
What the FDA Allows on Labels
U.S. food labels follow rules set out in the Code of Federal Regulations. Manufacturers can choose from several approved methods to calculate calories, including the general Atwater factors (4-4-9), the specific Atwater factors from USDA Handbook No. 74, or even bomb calorimetry data corrected by subtracting 1.25 calories per gram of protein to account for incomplete digestion.
Calories are rounded to the nearest 5-calorie increment up to 50 calories per serving, and to the nearest 10-calorie increment above that. Anything under 5 calories can be listed as zero, which is why a cooking spray with a tiny serving size can claim zero calories despite containing fat.
Fiber and sugar alcohols get special treatment. Because soluble non-digestible carbohydrates aren’t fully absorbed, they’re assigned 2 calories per gram instead of 4. Individual sugar alcohols each have their own assigned value: xylitol counts as 2.4 calories per gram, sorbitol as 2.6, maltitol as 2.1, and erythritol as zero. Manufacturers can subtract these from total carbohydrate before applying the standard factor, then add back the lower values.
Why Label Calories Don’t Always Match Reality
The Atwater system is a useful approximation, but it can’t capture the complexity of real digestion. One of the clearest examples comes from research on almonds. In a controlled feeding trial with 18 adults, researchers directly measured the metabolizable energy of almonds in different forms. Whole raw almonds provided 25% fewer calories than the Atwater factors predicted. Whole roasted almonds came in 19% lower, and chopped roasted almonds 17% lower. Almond butter, on the other hand, matched the Atwater prediction almost exactly, delivering 6.53 calories per gram versus the estimated 6.62.
The explanation comes down to physical structure. Harder, intact almonds fracture into fewer, larger particles during chewing, which limits how much fat your gut can access. When almonds are ground into butter, that fat is fully released and readily absorbed. This means the same food, prepared differently, can deliver meaningfully different amounts of energy to your body, something the standard calculation system doesn’t capture.
Similar patterns show up in other whole, fibrous, or minimally processed foods. Whole grains, seeds, and legumes with intact cell walls tend to yield fewer usable calories than their labels suggest, while highly processed foods with broken-down structures tend to match or even exceed their label values.
The Energy Your Body Actually Uses
Even after accounting for digestive losses, there’s one more step the Atwater system doesn’t include: the thermic effect of food, which is the energy your body spends digesting, absorbing, and processing nutrients. Protein costs the most to process, consuming 15 to 30% of its own calorie value during digestion. Carbohydrates require 5 to 10%, and fats just 0 to 3%.
This means a 200-calorie portion of chicken breast leaves you with noticeably fewer net calories than a 200-calorie portion of butter, even though the label treats both as equivalent. The Atwater system was never designed to account for this, and nutrition labels don’t reflect it. It’s one reason why calorie counts are best understood as reasonable estimates rather than precise measurements, useful for comparing foods and tracking intake, but not exact down to the last calorie.