Your body burns a fixed number of calories every day. Eat at that number and your weight stays stable. Eat below it and you lose. Eat above it and you gain.
The number is your TDEE (Total Daily Energy Expenditure). Most calorie calculators online estimate it. Some are accurate, most are not. This page explains the math behind a good calorie calculator, gives you the exact formula, and shows you how to apply it for fat loss, muscle gain, or maintenance.
Skip the math: use the free Virtus Athlete TDEE Calculator to get your BMR, TDEE, and goal target in seconds. The same Mifflin-St Jeor formula explained below.
The TDEE Formula
TDEE has two parts: your Basal Metabolic Rate (BMR) and an activity multiplier.
Step 1: Calculate BMR (Mifflin-St Jeor)
For men:
BMR = (10 × weight_kg) + (6.25 × height_cm) − (5 × age_years) + 5
For women:
BMR = (10 × weight_kg) + (6.25 × height_cm) − (5 × age_years) − 161
Mifflin-St Jeor is the most accurate BMR formula for the general population (about 5-10% margin of error). Older formulas like Harris-Benedict overestimate slightly.
Step 2: Multiply by Your Activity Level
| Activity Level | Description | Multiplier |
|---|---|---|
| Sedentary | Desk job, no exercise | BMR × 1.2 |
| Light | Light exercise 1-3 days/week | BMR × 1.375 |
| Moderate | Moderate exercise 3-5 days/week | BMR × 1.55 |
| Very Active | Hard exercise 6-7 days/week | BMR × 1.725 |
| Extra Active | Physical job + daily training | BMR × 1.9 |
The result is your TDEE: the number of calories you need per day to maintain your current weight.
How to Apply Your TDEE
| Goal | Daily Target |
|---|---|
| Mild fat loss | TDEE − 300 cal |
| Fat loss | TDEE − 500 cal |
| Maintenance | TDEE exactly |
| Lean muscle gain | TDEE + 200 cal |
| Muscle gain | TDEE + 300 cal |
For fat loss, 300 to 500 calories below TDEE is the sweet spot. Less than that and progress is too slow to stay motivated. The further below TDEE you go, the more important high protein and resistance training become.
For muscle gain, 200 to 300 above TDEE keeps fat gain minimal. Beginners and recompers can build muscle at maintenance or a slight deficit (see body recomposition).
Do Not Cut Too Aggressively
We strongly advise against very aggressive diets. A deficit larger than 500 calories per day is not a shortcut, it is a long detour. It triggers:
- Muscle loss alongside fat loss
- Drops in testosterone and thyroid output
- Loss of training performance and recovery
- Constant hunger, low energy, and rebound eating
- Reduced intake of essential vitamins, minerals, and fiber
The same principle applies on the surplus side. Adding more than 300 calories above TDEE piles on fat faster than it adds muscle, with no real upside for most lifters.
Nutrition Is Not Only About Weight
Hitting a calorie number is necessary, not sufficient. Food also delivers protein for muscle and recovery, fats for hormones, fiber for digestion, and vitamins and minerals for everything else your body does. Very aggressive deficits sacrifice all of this in pursuit of speed.
Eat enough food. Eat enough variety. A slower fat loss with full nutrition beats a fast one that wrecks your hormones, your strength, and your relationship with food. Use the TDEE calculator to find a sustainable target, then build meals around real foods that cover your protein, fats, fiber, and micronutrients.
Worked Example
A 30-year-old man, 80 kg, 180 cm, training 4 days a week:
BMR = (10 × 80) + (6.25 × 180) − (5 × 30) + 5
= 800 + 1125 − 150 + 5
= 1780 cal
Activity = moderate (4 sessions/week), so multiply by 1.55:
TDEE = 1780 × 1.55 = 2759 cal
To lose fat: eat ~2400 cal/day (TDEE − 350). To bulk: eat ~3000 cal/day (TDEE + 250).
Or drop those stats into the free TDEE calculator and you'll get the same numbers without doing the math by hand.
Why Calorie Calculators Differ
Plug your stats into 5 different calculators and you'll get 5 different numbers, sometimes 200+ calories apart. The differences come from:
- BMR formula used (Mifflin-St Jeor vs Harris-Benedict vs Katch-McArdle)
- Activity multiplier interpretation ("moderate" varies by site)
- Whether NEAT is included (Non-Exercise Activity Thermogenesis: walking, fidgeting)
Mifflin-St Jeor is the modern standard. If a calculator uses anything else, the result is less accurate.
Adjusting in Practice
The calculator is a starting point, not a final answer. Use it for the first 2-3 weeks, then adjust based on real-world results:
- Eat your calculated number for 2 weeks.
- Track weight daily, take a 7-day average.
- Compare week 1 average vs week 2 average.
- Fat loss target: 0.5-1% of body weight per week. Slower? Drop another 100-150 cal. Faster (and losing strength)? Add 100-150 cal.
- Muscle gain target: 0.25-0.5% per week. Faster usually means more fat gain than necessary.
Your TDEE also shifts as you lose or gain weight. Recalculate every 5-10 kg change.
