Calories Per Macro Gram Calculator
Turn grams of protein, carbs, fat, fibre and alcohol into total calories and see the percentage each macronutrient contributes.
How it works
kcal = protein·4 + (carbs−fibre)·4 + fibre·2 + fat·9 + alcohol·7 (Atwater factors)
The Atwater system assigns a fixed energy value to each gram of macronutrient: protein and carbohydrate at 4 kcal, fat at 9 kcal, and alcohol at 7 kcal. Fibre is a carbohydrate the body only partly digests, so it is counted at roughly 2 kcal per gram. Multiplying grams by these factors and summing gives a food’s or a day’s total energy, and dividing each part by the total shows the macro split.
Worked example
100 g protein (400 kcal) + 250 g carbs (1,000 kcal) + 70 g fat (630 kcal) = 2,030 kcal. Protein supplies 400 ÷ 2,030 ≈ 19.7%, carbs 49.3% and fat 31.0% of the day’s energy.
Sources: Atwater system
Frequently asked questions
Why is fat more than twice as calorie-dense?
Fat molecules are highly reduced (rich in carbon-hydrogen bonds), so oxidising them releases more energy per gram — about 9 kcal versus 4 kcal for protein and carbohydrate.
Does fibre really have calories?
Some. Soluble fibre is fermented by gut bacteria into short-chain fatty acids you absorb, yielding roughly 2 kcal per gram. Many labels approximate this by subtracting fibre from “net carbs”.
Do these factors apply to every food?
They are averages. Actual digestibility varies a little by food, and highly processed foods can be absorbed more completely than whole foods, but the Atwater factors are accurate enough for everyday tracking.
Related calculators
Educational estimate only — not medical advice, diagnosis or treatment. Consult a qualified clinician for health decisions.