"Eat 2,400 calories" is stated with more confidence than it deserves. Every TDEE number you have ever seen is an estimate stacked on an estimate — and the honest range around most people's true expenditure is ±15–20%. Here is where each formula earns or loses that error.
The four ways to get a number
| Method | Inputs | Typical error | Best for |
|---|---|---|---|
| Harris-Benedict (1919) | Sex, weight, height, age | +10–15% high on modern populations | Nobody, anymore |
| Mifflin-St Jeor (1990) | Sex, weight, height, age | ±10% on RMR | Default for everyone |
| Katch-McArdle | Lean body mass | ±8% if lean mass is accurate | DEXA-verified athletes |
| Doubly-labeled water | Isotope tracing | Gold standard | Researchers, not you |
The formula layer is the smaller error. The bigger one is the activity multiplier: "moderate" spans 1.5 to 1.7 in real people, and self-assessment skews optimistic by roughly half a level. A ±10% formula times a ±15% multiplier is your ±20% total.
The three-week calibration
Treat the calculated TDEE as a hypothesis, then measure:
- Eat the calculated TDEE for 14 days — weigh daily, same conditions (morning, post-bathroom, pre-food).
- Average week 1 vs week 2. Trend up more than ~0.3 kg? Your true TDEE is lower — subtract ~150 kcal per 0.25 kg of weekly gain. Trend down? The reverse.
- Hold the corrected number for another week to confirm flat trend. That corrected number is your real TDEE — better than any formula.
Water-weight noise is why we average: sodium cycles, glycogen, and fiber swings hide inside any single day. Two weekly averages cancel most of it.
What actually moves TDEE day to day
- NEAT — the sleeping giant. Non-exercise movement (walking, standing, fidgeting) varies between individuals by up to 2,000 kcal/day in controlled overfeeding studies — the largest single source of "fast metabolism" you will ever meet. A 6,000-step day vs 2,000-step day is a real meal's worth of energy.
- Training costs less than you think. An hour of lifting is 200–400 kcal; cardio machines overread 15–25%. The multiplier already covers it.
- Adaptation is real but slow. A long cut drops TDEE beyond what weight loss predicts (adaptive thermogenesis) — order 50–150 kcal after months of deficit. Reverse-diet weeks restore most of it.
Use our TDEE calculator for the hypothesis, run the three-week calibration, and you will out-know every formula — including this one.