How TDEE is calculated
The calculation has two steps. First, BMR is estimated using the Mifflin-St Jeor formula (sex, age, weight, height). Then an activity multiplier is applied — 1.2 for sedentary up to 1.9 for athletes — to account for energy spent on movement and thermogenesis.
The multipliers are approximations calibrated against doubly-labelled water studies (the gold standard for real-world calorie expenditure). At the population level they're reasonably accurate; individually, they can be off by ±15%. The most reliable way to find your true TDEE is to track intake and weight for 2–3 weeks at a stable weight and work backwards.
Choosing the right activity multiplier
This is where most people go wrong. Overestimating activity inflates your TDEE and slows fat loss without you understanding why.
- Sedentary (1.2×): desk job, mostly sitting, minimal walking. Under 5,000 steps per day.
- Lightly active (1.375×): 1–3 structured workouts per week. Normal walking as part of daily life.
- Moderately active (1.55×): 3–5 workouts per week, physically active enough to sweat regularly.
- Very active (1.725×): daily training or physically demanding work (construction, delivery, nursing).
- Athlete (1.9×): two-a-day training, professional athletes, or extremely high NEAT (non-exercise activity).
- Rule of thumb: if you're unsure, pick one level lower. You can always eat more; it's easier to diagnose a slow cut than an oversized TDEE estimate.
Setting a cut, maintenance, or bulk target
Your TDEE is your maintenance — eat this and your weight stays roughly stable. From there, the adjustment is straightforward.
For fat loss: subtract 300–500 kcal from TDEE. This creates a 0.3–0.5 kg/week deficit — sustainable and muscle-preserving at adequate protein intake. Don't go below 1,200 kcal (women) or 1,500 kcal (men) for extended periods regardless of what the math suggests.
For muscle gain: add 200–300 kcal above TDEE (a 'lean bulk'). Larger surpluses produce faster scale weight gains but more fat accumulation. Most natural lifters gain 0.5–1 kg of muscle per month maximum, so a 300 kcal surplus is plenty.
Why TDEE changes over time
TDEE isn't a fixed number — it shifts with your body.
- Weight loss: every kilogram lost reduces BMR by ~8–12 kcal/day. Lose 10 kg and your TDEE is meaningfully lower. Recalculate.
- Muscle gain: adding lean mass raises BMR. A 5 kg muscle gain may add 65–70 kcal/day to TDEE.
- Metabolic adaptation: prolonged large deficits (>25%) suppress metabolic rate beyond what weight loss alone explains. This is the 'starvation mode' effect — real but smaller than commonly claimed (typically 5–10% suppression).
- Age: BMR declines slowly with age due to lean mass loss. Staying active and resistance training is the most effective way to slow this.