Worked example: 2,259 kcal at 80 kg, cutting
| Macro | Grams | Calories | Share | How it was set |
|---|---|---|---|---|
| Protein | 176 g | 704 | 31% | 2.2 g per kg of bodyweight |
| Fat | 63 g | 567 | 25% | 25% of calories, above the 40 g floor |
| Carbohydrate | 247 g | 988 | 44% | The remainder |
Protein at 4 kcal a gram, fat at 9, carbohydrate at 4. The three add back to 2,259.
Why protein is set first, and not as a percentage
This is the one real decision on the page. Almost every macro calculator asks you to pick a split — 40/30/30, or 50/25/25 — and then applies those percentages to your calories. That is backwards, and it produces obviously wrong answers at both ends of the weight range.
Take 30% protein. On 1,600 kcal that is 120 g, which is too little for a 100 kg person trying to hold muscle in a deficit. On 3,200 kcal it is 240 g, which is more than almost anyone needs and is occupying calories that carbohydrate would use better. The same percentage, two wrong answers, because the requirement scales with bodyweight and the percentage scales with calories.
So: protein comes from bodyweight, using the same ranges as the protein intake calculator. Fat is a share of calories, because fat requirement genuinely does scale with total intake — but with a floor. Carbohydrate takes the remainder, because carbohydrate is the macro with no minimum requirement and the largest performance effect at the margin.
The fat floor, and why it exists
Below roughly 0.5 g of fat per kilogram of bodyweight, sustained, you are into territory that causes problems — fat-soluble vitamin absorption and hormone production both depend on it. A percentage split walks into this without ever mentioning it: 20% fat on an aggressive cut at 1,500 kcal is 33 g, which for a 100 kg person is well under the floor.
When the percentage you chose would land below the floor, this calculator raises fat to the floor and says so, and carbohydrate absorbs the difference. If that pushes carbohydrate below zero — protein and fat alone exceeding the calorie budget, which is reachable on an aggressive cut at a high bodyweight — the page tells you the target is not achievable rather than printing a negative number.
Methodology
Protein grams = bodyweight × a coefficient from goal and training status (1.6–2.2 g/kg for resistance training, top of the range while cutting). Fat grams = calories × fat share ÷ 9, raised to bodyweight × 0.5 if that is higher. Carbohydrate grams = (calories − protein kcal − fat kcal) ÷ 4. Energy densities are the conventional 4, 9 and 4 kcal per gram. Percentages shown are each macro's calories as a share of the total, computed after rounding, so they can sum to 99 or 101.
Limitations
Total calories decide weight change; macros decide what the weight change is made of and how you feel doing it. Getting the split slightly wrong is a much smaller error than getting the total wrong, and both are smaller than the error in your maintenance estimate, which is roughly ±10% before you have collected any of your own data.
The 4/9/4 energy densities are conventional rounded figures. Real absorbed energy varies with food processing, fibre content and the individual, by a few per cent in each direction. Nobody's diet is accurate to the gram and this one is not either.
Fibre and alcohol are not modelled. Fibre is counted inside carbohydrate here, which is the convention on food labels in the United States but not everywhere; alcohol carries about 7 kcal a gram and is simply absent from the split. If you drink, those calories come out of the carbohydrate and fat allowance in practice.
The protein ranges come from research on people doing resistance training. Endurance athletes, older adults and people not training at all sit outside that population, and the alternative ranges offered for them are less well evidenced — they are marked as such on the protein page too.
On the calorie total itself: if the figure you enter is below roughly 1,500 kcal a day for a man or 1,200 for a woman, this page will say so above the result. Those are the conventional lower bounds below which a general-population tool has no business sending anyone — below them it becomes difficult to meet micronutrient requirements from food at all, and no arrangement of the three macros fixes that. The right response is a smaller deficit and a longer timeline, or a conversation with a registered dietitian or your doctor.
This is a fitness estimate, not medical advice, and not a substitute for a professional who can actually see you. If you have a condition affected by what you eat — kidney disease, diabetes, anything requiring a managed diet — the split belongs with a doctor or a dietitian, not with a web page.