The Mifflin-St Jeor formula, activity multipliers, and why the 3,500-calorie rule oversimplifies weight loss. A realistic look at daily calorie burn.
Basal Metabolic Rate (BMR) is the energy your body expends at complete rest, in a thermoneutral environment, after an overnight fast. It is the cost of staying alive: breathing, circulating blood, maintaining ion gradients, and keeping cells functioning. The National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) estimates that BMR accounts for roughly 60-75% of total daily energy expenditure in most sedentary adults, which is why two people of the same weight can burn very different numbers of calories.
Total Daily Energy Expenditure (TDEE) is the full 24-hour calorie burn. It is the sum of BMR, the thermic effect of food (TEF, the energy used to digest and absorb meals, about 10% of total intake), exercise activity thermogenesis, and non-exercise activity thermogenesis (NEAT) such as walking, fidgeting, and standing. Weight is stable when calories consumed equal TDEE; a sustained surplus causes gain and a sustained deficit causes loss. This is the energy balance equation that underlies every credible weight-management guideline.
BMR cannot be measured without a laboratory (indirect calorimetry), so it is estimated from equations. The most widely recommended is the Mifflin-St Jeor equation, published in 1990. In 2005 the American Dietetic Association (now the Academy of Nutrition and Dietetics) reviewed the published prediction equations and concluded that Mifflin-St Jeor produced the most accurate results for both healthy and obese adults, with typical error within about 10% of measured values.
BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + s (s = +5 for men, −161 for women)
| Prediction Equation | Notes |
|---|---|
| Mifflin-St Jeor (1990) | Preferred by the Academy of Nutrition and Dietetics; ~10% typical error |
| Harris-Benedict (revised 1984) | Tends to overestimate by ~5% in modern populations |
| Katch-McArdle | Uses lean body mass; most accurate for very lean or muscular individuals |
TDEE is most simply estimated by multiplying BMR by an activity factor. The multipliers below come from the Harris-Benedict convention and remain the standard shorthand used in clinical and fitness settings. They are population averages, not exact, because two people who both "exercise 3 days a week" can differ widely in NEAT and training intensity.
| Activity Level | Multiplier (BMR ×) |
|---|---|
| Sedentary (little or no exercise) | 1.2 |
| Lightly active (1-3 days/week) | 1.375 |
| Moderately active (3-5 days/week) | 1.55 |
| Very active (6-7 days/week) | 1.725 |
| Extra active (physical job + training) | 1.9 |
A common error is selecting a multiplier based on planned exercise rather than actual lifestyle. Someone who trains hard for an hour but sits for the other 23 hours is often closer to "lightly active" than "very active," because NEAT (standing, walking, household movement) typically contributes more to daily burn than the workout itself. Track both planned exercise and general movement before choosing a category.
Consider a 35-year-old man who is 5 ft 10 in (178 cm) and weighs 180 lb (81.6 kg). Applying Mifflin-St Jeor: BMR = 10 × 81.6 + 6.25 × 178 − 5 × 35 + 5 = 816 + 1,112.5 − 175 + 5 = 1,759 kcal/day. This is the energy to simply exist; it already covers about two-thirds of his daily burn before any movement.
If he exercises moderately (3-5 days per week), his TDEE is 1,759 × 1.55 = 2,726 kcal/day. The same person at a sedentary desk job would be at 1,759 × 1.2 = 2,111 kcal/day, and as a very active athlete at 1,759 × 1.725 = 3,034 kcal/day. The activity multiplier therefore swings his maintenance calories by more than 900 kcal per day, which is why training volume is the largest controllable lever in energy balance.
To lose approximately 1 lb per week, the conventional guidance is a 500 kcal/day deficit, bringing his intake to 2,726 − 500 = 2,226 kcal/day. To gain weight, a comparable surplus is applied. The Metriova BMR calculator performs the Mifflin-St Jeor calculation and applies the activity multiplier automatically from your inputs.
The rule that 1 lb of body fat equals roughly 3,500 kcal traces to a 1958 estimate by Max Wishnofsky and remains the basis for the 500 kcal/day deficit guideline. It is a useful starting point, but the NIDDK and recent metabolic modeling caution that it oversimplifies reality. As weight drops, BMR falls (less mass to maintain) and the body becomes more energy-efficient, so a fixed 500 kcal deficit produces diminishing weekly losses over time.
A dynamic model developed by NIH researchers (Hall et al., 2011) predicts that the true 1 lb/week pace requires a larger deficit than 500 kcal as the diet progresses, and that the first weeks of loss include water and glycogen rather than fat alone. The practical takeaway: treat the 3,500 kcal rule as an initial estimate, recompute TDEE as body weight changes, and expect real-world loss to run 15-25% slower than the linear rule predicts over months.
Several factors cause estimated TDEE to diverge from measured expenditure, and accounting for them improves any plan:
Sources: National Institute of Diabetes and Digestive and Kidney Diseases (energy balance and weight management), Academy of Nutrition and Dietetics (Mifflin-St Jeor evidence review, 2005), Mifflin et al. (1990, original equation), Hall et al. (NIH, dynamic weight-loss model, 2011), and the American Council on Exercise. This content is educational and is not medical advice; consult a registered dietitian or physician for an individualized plan.
BMR & TDEE Calculator — Calories Your Body Burns
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