Every weight-change plan starts from one estimate: how many calories you burn in a day. That number is your Total Daily Energy Expenditure (TDEE), and it's built from your Basal Metabolic Rate — the energy your body uses just to stay alive — plus everything you do on top of it. No formula can measure it exactly, but a good estimate gets you close enough to start, and the scale will correct the rest.
From BMR to TDEE
The Mifflin-St Jeor equation is the most widely used BMR formula: for men, 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5; for women, the same with −161 instead of +5. For a 30-year-old man at 80 kg and 180 cm, that's 800 + 1125 − 150 + 5 = 1,780 kcal/day — the energy cost of existing.
TDEE multiplies BMR by an activity factor: 1.2 for sedentary desk work, 1.375 for light exercise 1–3 days a week, 1.55 for moderate exercise 3–5 days, 1.725 for heavy training, up to 1.9 for very physical lifestyles. The same man at moderate activity: 1,780 × 1.55 ≈ 2,760 kcal/day. Eat around that and weight stays roughly flat; eat below it and weight trends down.
Activity factors are the weak link
The multiplier is a guess, and most people overestimate their activity level. If you track calories for two weeks and weight doesn't move as expected, the activity factor — not the BMR formula — is the usual culprit. Adjust down by one step and re-test.
Where estimates drift
- Body composition — formulas can't see muscle mass, so they overestimate needs for muscular people and underestimate for very sedentary ones.
- Individual variance — real BMR varies ±5–10% between people with identical inputs.
- Weight change — as you lose weight, your BMR falls too, so a static TDEE becomes stale within weeks.
- Exercise calorie burn — calorie-burn figures for workouts are themselves estimates, not measurements.
The practical approach: use the estimate as your starting budget, track weight over 2–4 weeks, and adjust by 200–300 kcal in the direction you need. The formula gets you close; the scale gets you exact.
Estimate your TDEE
Health Metrics Studio calculates BMR, TDEE, macros, and healthy weight range in your browser — no accounts, no uploads.
Open Health Metrics StudioWhy calculators default to Mifflin-St Jeor now
The original BMR formula in wide use, Harris-Benedict, dates to 1919 and was revised once in 1984. Mifflin-St Jeor, published in 1990, was developed specifically because validation studies found Harris-Benedict was overestimating resting metabolic rate for a modern population by a meaningful margin — commonly cited at around 5%, more for people with higher body fat percentages, since the older formula was built from a study population that was on average leaner and more muscular than today's. Later comparison studies have repeatedly found Mifflin-St Jeor the more accurate of the two for most people, which is why it became the default in clinical and consumer tools alike. Neither formula is exact for any individual — they're both population averages — but Mifflin-St Jeor's error is smaller and more evenly distributed.
The part no static formula can model: metabolic adaptation
A sustained calorie deficit doesn't just lower weight — it lowers the metabolic rate itself, beyond what the drop in body mass alone would predict. This effect, sometimes called adaptive thermogenesis, means a body that has been dieting for months burns fewer calories at the same weight than a body that arrived at that weight without dieting. It's a real, measured physiological response, not just a matter of moving less. The practical consequence is that a TDEE estimate calculated once at the start of a diet becomes progressively less accurate the longer the deficit continues — not only because weight changed, but because the body's efficiency changed too. This is the main reason long diets often plateau even when the math on paper still says they shouldn't.
NEAT: the biggest source of variation the multiplier ignores
Non-Exercise Activity Thermogenesis — fidgeting, posture, standing versus sitting, the countless small movements of an ordinary day that have nothing to do with deliberate exercise — varies enormously between individuals, and research has measured differences of over 1,000 kcal per day between two people of similar size and formal activity level, driven almost entirely by how naturally restless or still someone is. Activity multipliers only account for structured exercise ("3–5 days a week"), not this background variation, which is a large part of why two people who report identical workout routines can have genuinely different real-world TDEE. It's also one of the more plausible reasons someone finds themselves burning noticeably more or less than a calculator predicts even after accounting for measurement error elsewhere.