How your macros are set
The calculation runs in three steps. First it estimates your maintenance calories using the Mifflin-St Jeor equation and your activity level — the same method as the TDEE calculator. Then it adjusts for your goal: about −20% to lose fat, +12% to build muscle. Then it divides those calories:
- Protein: around 2 g per kg of body weight. Each gram provides 4 calories.
- Fat: about 25% of total calories. Each gram provides 9 calories.
- Carbohydrate: whatever calories remain. Each gram provides 4 calories.
Worked example: a 30-year-old man, 75 kg and 180 cm, moderately active. His BMR is 10×75 + 6.25×180 − 5×30 + 5 ≈ 1,730 kcal, and his maintenance is 1,730 × 1.55 ≈ 2,682 kcal. That splits into 150 g protein (2 g/kg, 600 kcal), 75 g fat (25% of calories, 671 kcal), and the remaining 1,411 kcal as carbohydrate — about 352 g. Enter those numbers above and you should get the same result.
Which parts of this are actually evidence-based
Worth separating, because most macro calculators present all three numbers with equal confidence and they do not deserve it.
The protein target is well supported. The range used here sits within the International Society of Sports Nutrition position stand, which recommends roughly 1.4–2.0 g/kg for building and maintaining muscle in people who train, with higher intakes justified during a calorie deficit. A meta-analysis in the British Journal of Sports Medicine found protein supplementation meaningfully improved strength and muscle gains from resistance training, with benefits plateauing somewhere around 1.6 g/kg for most people.
The fat and carbohydrate split is a convention, not a requirement. The 25% fat share is a practical default that keeps intake comfortably above the level needed for hormone production and fat-soluble vitamin absorption, without crowding out the carbohydrate that fuels training. There is no strong evidence that a particular carb-to-fat ratio is superior for body composition once calories and protein are matched. If you feel better on more fat and fewer carbs, or the reverse, that preference is a legitimate reason to adjust.
Why protein comes first
- It protects muscle in a deficit. When you are eating less than you burn, adequate protein plus resistance training is what determines whether the weight you lose is mostly fat or partly muscle.
- It is the most satiating macronutrient, gram for gram, which makes any calorie target easier to hold.
- It has the highest thermic effect — roughly 20–30% of protein calories are spent digesting and processing it, against about 5–10% for carbohydrate and 0–3% for fat.
Spreading protein across three or four meals of roughly 25–40 g appears to work slightly better for muscle protein synthesis than concentrating it in one sitting, though total daily intake matters most.
Adjusting the split for yourself
Hit protein, then treat fat and carbohydrate as adjustable within sensible floors. Keep fat above roughly 0.5 g/kg to protect hormonal function and vitamin absorption. Keep carbohydrate high enough to fuel your training — if your sessions are deteriorating and you feel flat, low carbohydrate intake is a common culprit and an easy thing to test.
Endurance athletes generally need more carbohydrate than this default provides. People doing mostly low-intensity activity can comfortably run lower.
Make it practical
You do not need to hit these numbers exactly. Getting protein close to target and keeping fat and carbohydrate roughly in range captures nearly all the benefit; agonising over the last few grams captures almost none. Judge whether the plan is working over two to three weeks of consistent weight data, and adjust calories before you start rearranging ratios.
Tracking everything forever is also not the goal. Most people find that a few weeks of measuring teaches them what portions actually look like, after which they can estimate well enough to maintain without weighing food.