How macros are calculated
The calculator splits your daily calories across the three macronutrients according to your goal and converts each into grams. Every gram of protein and carbohydrate provides 4 kcal, and every gram of fat 9 kcal (the Atwater factors).
The splits used: for weight loss 30% protein, 40% carbs, 30% fat; for maintenance 25% protein, 45% carbs, 30% fat; for muscle gain 25% protein, 50% carbs, 25% fat. For example, 2,000 kcal with a weight loss goal works out to about 150 g protein, 200 g carbs and 67 g fat a day.
Why these ratios?
Keeping protein high while losing weight reduces muscle loss and increases fullness, which is why the weight loss split has the largest protein share. When building muscle, the carbohydrate share goes up to fuel training. Fat is needed for hormone production and absorbing fat-soluble vitamins, so it should not drop below about 20% of calories.
The ratios are based on the acceptable ranges health authorities recommend for adults: protein 10-35%, carbohydrate 45-65% (slightly below when losing weight), fat 20-35%.
Check protein against your body weight
Percentages are practical, but protein needs really depend on body weight. The minimum for sedentary adults is 0.8 g per kg (0.36 g per lb); for people losing weight or training regularly, 1.6-2.2 g per kg (0.7-1 g per lb) is more effective at preserving muscle.
When you enter your weight, the calculator shows protein per kilogram. If it is well above 2.2 g/kg, you can shift some of that share to carbohydrates; if it is below 1.2 g/kg, especially while losing weight, consider raising it.
Limitations
Macro splits do not override energy balance: total calories are what drive weight change. These ratios are general starting points for healthy adults; ketogenic or very low-carb diets, vegetarian eating or endurance sports may call for different splits.
The source of your macros matters too: getting the same grams of carbohydrate from whole grains, vegetables and legumes provides far more fiber, vitamins and fullness than getting them from sugary foods.
When to see a doctor
High protein intake can be harmful if you have kidney disease, and if you have diabetes your carbohydrate intake should be planned together with your medication or insulin. During pregnancy or breastfeeding, with a history of disordered eating or with a chronic condition, set your macro targets with a registered dietitian.