Grams to Calories Calculator — Macronutrients

Formula

How this is calculated

Food energy is calculated using the Atwater general factors, the system adopted by most food labelling authorities worldwide. Each macronutrient class delivers a fixed number of kilocalories per gram: protein 4 kcal/g, carbohydrates 4 kcal/g, dietary fat 9 kcal/g and pure ethanol 7 kcal/g. Total energy is the sum across all macronutrients you enter.

Fat has more than twice the energy density of protein or carbohydrate because fat molecules are almost entirely carbon and hydrogen, while protein and carbohydrate molecules also contain oxygen, which does not combust to release energy. Alcohol sits between the two — more energy-dense than protein and carbs, but less than fat.

Note that dietary fibre (a carbohydrate) is only partially fermented by gut bacteria, so it delivers fewer calories in practice (roughly 2 kcal/g); food labels often subtract fibre from total carbohydrates before applying the × 4 factor. This calculator uses the simple, published macronutrient totals. The Atwater factors are averages — actual absorption efficiency varies slightly by food matrix, individual and processing.