How this calculator works
Food energy is estimated from the amounts of carbohydrate, protein, fat and alcohol. The Atwater system assigns average energy values of 4 kilocalories per gram to carbohydrate and protein, 9 to fat and 7 to alcohol. These are averages across mixed foods, not exact values for every molecule: fibre, sugar alcohols and different fatty acids have their own factors, and food labels may use slightly different rounded figures. Alcohol is not a nutrient, but it does supply energy, and it is included here only when you choose to enter it.
Formula
calories (kcal) = carbohydrate grams x 4 + protein grams x 4 + fat grams x 9 + alcohol grams x 7. For a drink, alcohol grams = volume in mL x ABV percent / 100 x 0.789 g/mL, where 0.789 g/mL is the density of ethanol.
Worked example
30 g carbohydrate, 20 g protein and 10 g fat give 120 + 80 + 90 = 290 kcal. A 355 mL beer at 5 percent alcohol contains about 14.0 g of alcohol, adding about 98 kcal. One gram of pure fat is 9 kcal, while one gram of pure carbohydrate or protein is 4 kcal.
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Frequently asked questions
Are these the calories on a nutrition label?
They use the same Atwater factors for the three macronutrients, so they should be close. Labels round to the nearest 5 or 10 calories, and they may treat fibre, sugar alcohols or specific fats differently.
Why does alcohol count as 7 calories per gram?
Ethanol is metabolised for energy even though it is not an essential nutrient. The accepted average is about 7 kcal per gram, between carbohydrate at 4 and fat at 9.
Is this a diet or medical calculator?
No. It is an arithmetic conversion of grams to energy using population-average factors. Individual needs depend on body size, activity and health, and a dietitian or physician is the right source for personal advice.