One gram of carbohydrate contains 4 calories. This is the standard conversion factor used on every nutrition label in the United States and in dietary guidelines worldwide. So if a food has 30 grams of total carbohydrates, those carbs alone contribute about 120 calories.
Where the 4-Calorie Figure Comes From
The 4-calories-per-gram value dates back to the late 1800s, when a chemist named W.O. Atwater and his colleagues at the USDA measured how much energy the body actually extracts from food. They burned food in a device called a calorimeter, then adjusted downward to account for the fact that humans don’t absorb 100% of what they eat. The resulting figures, known as the Atwater general factors, assigned 4 calories per gram to both carbohydrates and protein, and 9 calories per gram to fat. These same conversion factors are still required by the FDA for nutrition labeling today.
Not All Carbs Deliver the Full 4 Calories
The 4-calorie rule applies to digestible carbohydrates: sugars and starches that your body breaks down and absorbs. But the “total carbohydrate” line on a nutrition label also includes fiber and sugar alcohols, which your body handles differently.
Fiber passes through your digestive system mostly intact. The FDA assigns soluble fiber (the type that partially ferments in your gut) a value of 2 calories per gram, and insoluble fiber contributes essentially nothing. Sugar alcohols, commonly found in sugar-free candy and protein bars, fall somewhere between 0 and 3 calories per gram depending on the type. Erythritol, for example, counts as 0 calories per gram, while sorbitol counts as 2.6.
This is why “net carbs” became a popular concept, especially in low-carb dieting. The basic idea: subtract fiber and sugar alcohols from total carbohydrates to estimate the carbs that actually raise your blood sugar and deliver the full caloric load. A protein bar listing 24 grams of total carbs but containing 10 grams of fiber and 8 grams of sugar alcohols would have only 6 net carbs, representing a meaningfully lower calorie contribution than the total carb number suggests.
How Carb Calories Compare to Fat and Protein
Carbohydrates and protein both provide 4 calories per gram, making them calorically identical by weight. Fat is more than twice as calorie-dense at 9 calories per gram. Alcohol, though not a macronutrient, provides 7 calories per gram.
Your body also burns some energy just digesting food, a process called the thermic effect. Carbohydrates use up about 5 to 10% of their calories during digestion and metabolism. Protein has a much higher thermic effect (20 to 30%), which is one reason high-protein diets are often recommended for weight management. Fat has the lowest thermic cost at around 0 to 3%. So while a gram of carbohydrate technically contains 4 calories, the net energy your body keeps is slightly less.
Putting It Into Practice
If you’re counting calories or tracking macros, the math is straightforward. Multiply the grams of carbohydrate in a food by 4 to get the calories from carbs. A medium banana with about 27 grams of carbs contributes roughly 108 calories from carbohydrates alone. A slice of white bread with 13 grams of carbs adds about 52 carb calories.
When reading nutrition labels, keep in mind that the total calorie count already factors in carbs, protein, and fat together. You don’t need to add carb calories on top of the listed total. The per-gram conversion is most useful when you’re calculating macronutrient ratios or when a label lists grams but not a calorie breakdown by nutrient.