Macro Calculator

How this is calculated

Two steps. First your daily calorie target, using the Mifflin-St Jeor equation and an activity multiplier — the same maths as the BMR and TDEE calculator, adjusted by your goal. Then that total is divided into macronutrients using an evidence-informed split:

Grams are derived using the Atwater factors: 4 calories per gram of protein, 4 per gram of carbohydrate, and 9 per gram of fat. These are the same constants the Diet Science app uses when it sets a logged-in user's daily targets, read from a single shared source so the two can never disagree.

Why these splits

Protein rises in both the fat-loss and muscle-gain splits, for different reasons. In a deficit it protects lean mass and improves satiety; in a surplus it supplies the substrate for building tissue. Fat rises slightly during fat loss because very low fat intakes tend to hurt adherence and hormone production.

Carbohydrate takes the remainder. That's deliberate — for most people it's the lever with the most room to move, and it's where individual response varies most. If you have insulin resistance, both the amount and the form matter more than the percentage suggests; see the insulin resistance guide.

Cross-check your protein number

A percentage-of-calories split can produce an odd protein figure at the extremes — very low calorie targets under-deliver protein, and very high ones overshoot it. Protein needs scale with body weight, not with energy intake.

Run the protein calculator as well, and if the two disagree, trust the grams-per-kilogram figure and adjust carbohydrate and fat around it.

Using the numbers without becoming obsessive

Macro targets are a planning tool, not a daily exam. Consistently landing within about 10% of your protein target and roughly in range on the others produces essentially the same result as hitting them exactly — and it's far more sustainable.

If tracking starts driving anxiety, or a missed target feels like a failure rather than a Tuesday, stop tracking and speak to your GP. That pattern matters more than any macro split.