Every nutrition plan starts with one number: how much energy your body uses in a day. Get it roughly right and the rest becomes simple.
Step 1: Your resting energy (BMR)
FitAI Coach uses the Mifflin-St Jeor equation, which research has found to be among the most accurate for adults:
- Men: 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5
- Women: 10 × weight (kg) + 6.25 × height (cm) − 5 × age − 161
A 30-year-old woman who is 160 cm and 64 kg has a BMR of about 1,329 kcal. That's what her body burns lying still all day.
Step 2: Add your activity (TDEE)
Multiply BMR by an activity factor: 1.2 if you mostly sit, 1.375 if you exercise 1 to 3 days a week, 1.55 for 3 to 5 days, 1.725 for hard training most days. Our example woman, exercising two or three times a week (factor 1.375), lands around 1,830 kcal.
Step 3: Adjust for your goal
- Fat loss: eat about 450 kcal below your TDEE. That's a steady loss of around 0.4 kg a week without crashing your energy.
- Muscle gain: eat about 250 kcal above. A small surplus limits fat gain.
- Fitness or maintenance: eat at your TDEE.
FitAI Coach never sets a target below 1,200 kcal for women or 1,500 kcal for men. Going lower makes it hard to get enough protein and nutrients, and you will feel it in training.
Step 4: Split into protein, carbs and fat
Protein comes first: about 1.8 g per kg of bodyweight when losing fat or building muscle, 1.4 g otherwise. Fat takes roughly 27% of calories. Carbohydrates fill the rest and fuel your training.
When to recalculate
Treat the number as a starting point. Weigh yourself a few mornings a week and look at the two-week average. If it hasn't moved in the direction you want after three weeks, adjust by 100 to 150 kcal. Recalculate whenever your weight changes by 3 kg or more.