Personalised macros,
tuned to your goal.

Field guide

How a macro target is built.

Macronutrients — protein, carbohydrate, and fat: are the three energy-providing components of food. They behave very differently in your body, so dialing the split lets you target composition outcomes (muscle gain, fat loss, recomposition) at the same calorie level. The pipeline this calculator runs is the standard four-step approach used by sports-nutrition coaches.

Step 1: BMR via Mifflin-St Jeor

Basal Metabolic Rate is the calories your body burns at complete rest. The Mifflin-St Jeor equation (1990) is the modern default — it consistently outperforms the older Harris-Benedict formula against indirect-calorimetry data on healthy adults.

Step 2: TDEE = BMR × activity

Total Daily Energy Expenditure scales BMR by an activity multiplier that captures the calories you burn from non-exercise movement and structured training. Five levels cover most lifestyles:

• Sedentary (×1.2): desk job, no exercise
• Lightly active (×1.375): 1–3 days/week light exercise
• Moderately active (×1.55): 3–5 days/week moderate exercise
• Very active (×1.725): 6–7 days/week hard exercise
• Extra active (×1.9): hard exercise + physical job

Step 3: Calorie target by goal

Your daily target adjusts TDEE up or down depending on whether you're cutting, maintaining, or gaining:

• Lose weight: TDEE × 0.80 (~20% deficit, about 1 lb / 0.45 kg per week of loss).
• Maintain: TDEE × 1.00.
• Build muscle: TDEE × 1.10 (~10% surplus, about 0.5 lb / 0.23 kg per week of lean gain).

Step 4: Macros by body weight, not by ratio

A common mistake is to apply a fixed macro percentage (e.g. 40/30/30) regardless of body size. Two problems with that: a 130-lb woman and a 220-lb man on the same percentage get very different absolute protein, even though their per-pound needs are similar and a deep cut at a fixed ratio leaves protein dangerously low. The sports-nutrition standard pins protein and fat to body weight and lets carbs fill the rest:

• Lose: 1.1 g/lb (≈2.4 g/kg) protein, 0.40 g/lb (≈0.9 g/kg) fat
• Maintain: 0.8 g/lb (≈1.8 g/kg) protein, 0.45 g/lb (≈1.0 g/kg) fat
• Gain: 1.0 g/lb (≈2.2 g/kg) protein, 0.40 g/lb (≈0.9 g/kg) fat

Energy density: protein and carbs are 4 kcal/g each, fat is 9 kcal/g. The calculator converts the per-pound prescriptions into grams, multiplies through by 4 or 9 for kcal, and the leftover calories go into carbs.

Worked example: 30-year-old male, 175 lb, 5'10", moderate, maintain

• BMR (Mifflin-St Jeor) ≈ 1,725 kcal
• TDEE = 1,725 × 1.55 ≈ 2,674 kcal
• Calorie target (maintain) ≈ 2,674 kcal
• Protein = 175 × 0.8 = 140 g (560 kcal)
• Fat = 175 × 0.45 ≈ 79 g (708 kcal)
• Carbs = (2,674 − 560 − 708) ÷ 4 ≈ 352 g (1,406 kcal)

Why the protein floor matters in a deficit

When you're in an energy deficit, your body breaks down both fat and lean tissue. A high-protein intake (≥1.0 g/lb) plus resistance training is the most reliable way the literature has identified to keep the lost weight as fat rather than muscle. Drop protein and the scale still moves , but you arrive at a lighter, weaker version of yourself.

Why fat shouldn't go below ~0.3 g/lb

Dietary fat supports steroid-hormone production and the absorption of fat-soluble vitamins (A, D, E, K). Going below about 0.3 g/lb for prolonged periods disrupts hormones, particularly testosterone in men and menstrual cycles in women. The cut prescription this calculator uses (0.40 g/lb) sits comfortably above that floor.

Validation rules used here

• Age must be between 10 and 100.
• Weight: 25–300 kg (≈ 55–660 lb).
• Height: 100–250 cm (≈ 3'3" – 8'2").

Outside those ranges the calculator surfaces an explicit error rather than silently spitting out a nonsense plan.

Disclaimer

This is a starting point, not a prescription. Mifflin-St Jeor is accurate to roughly ±10% for the general adult population — it does not account for medical conditions, medications, pregnancy, or extreme body compositions. If you're managing a clinical condition or chasing competitive performance, work with a registered dietitian who can adjust against your real-world measurements (weigh-ins, training logs, biofeedback).