Multiply grams by the nutrient's energy factor
Each macronutrient has a fixed calories-per-gram value, the Atwater factor. Protein and carbohydrate are 4, fat is 9, alcohol is 7. Multiply the grams of each by its factor and add them up. To go the other way, divide calories by the factor to get grams.
Gram for gram, fat is more than twice the energy
This is the number people underestimate. The same 100 grams of fat carries as many calories as about 225 grams of carbohydrate, which is why fatty foods add up so fast.
Same mass, very different energy. Fat delivers 2.25× the calories of protein or carbohydrate, and alcohol sits between them.
Quick reference and mental math
| Macronutrient | Per gram | Per 25 g | Per 100 g |
|---|---|---|---|
| Protein | 4 kcal | 100 kcal | 400 kcal |
| Carbohydrate | 4 kcal | 100 kcal | 400 kcal |
| Fat | 9 kcal | 225 kcal | 900 kcal |
| Alcohol | 7 kcal | 175 kcal | 700 kcal |
Shortcuts: 25 g of protein or carbs is 100 kcal, and 10 g of fat is about 90 kcal. A tablespoon of olive oil (14 g of fat) is roughly 120 kcal.
Two easy slips
- Don't forget alcohol. At 7 kcal per gram it is nearly as dense as fat. A 25 ml shot of 40 percent spirit holds about 10 g of pure alcohol, so around 70 kcal before any mixer.
- Labels round. Manufacturers round to the nearest few calories, and fiber is often counted at 2 kcal per gram rather than 4, so a label total can differ slightly from the raw 4-4-9 math.
Common questions
How many calories are in a gram of protein, carbs and fat?
Protein and carbohydrate are both 4 calories per gram, fat is 9, and alcohol is 7. These are the Atwater factors used on nutrition labels worldwide.
Why does fat have more than twice the calories of carbs?
Fat molecules pack more chemical energy per gram than protein or carbohydrate. Burned in a calorimeter, a gram of fat releases about 9 calories against 4 for the others, roughly 2.25 times as much.
Do I multiply the whole food weight or just the macronutrient?
Just the macronutrient. A 100 gram chicken breast holds about 31 grams of protein, not 100. The rest is mostly water. Multiply the grams of each nutrient, not the total food weight.
