How Many Calories Should I Eat? A Band, Not a Number

The question “how many calories should I eat” has a calculable answer, but it is not a single number. Resting metabolic rate (RMR) can be estimated with the Mifflin-St Jeor equation, and total daily energy expenditure (TDEE) is RMR multiplied by an activity factor. Both steps carry error. The published standard error for Mifflin-St Jeor is roughly 10% in adults, which for a typical 2,300 kcal maintenance figure is about 230 kcal. That means the honest answer is a band: roughly 2,070 to 2,530 kcal. The right use of the estimate is to start in the middle and let two weeks of scale readings say which end you are.

The equation and its inputs

Mifflin-St Jeor (1990) predicts RMR from body mass, height, and age. It is the equation most dietetics associations recommend for non-obese adults because it outperforms Harris-Benedict in validation samples.

For men: RMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age (y) + 5 For women: RMR = 10 × weight (kg) + 6.25 × height (cm) − 5 × age (y) − 161

Weight is in kilograms; divide pounds by 2.2046. Height is in centimetres; multiply inches by 2.54. The output is kilocalories per day at rest.

Total daily energy expenditure is RMR times an activity multiplier. The multipliers below are the conventional ones, but they are the weakest link in the chain. They are not measured for the individual; they are population averages for self-reported activity levels, and self-reported activity is systematically overestimated.

The multiplier is a source of error separate from the RMR equation. A person who selects “moderately active” but actually averages two sessions a week is closer to 1.375. The difference between 1.375 and 1.55 on a 1,600 kcal RMR is 280 kcal per day.

A worked calculation

Use a 40-year-old woman, 165 cm, 70 kg, who exercises three days a week and selects “moderately active.”

RMR = 10 × 70 + 6.25 × 165 − 5 × 40 − 161
RMR = 700 + 1031.25 − 200 − 161
RMR = 1370.25 kcal/day

TDEE = RMR × 1.55
TDEE = 1370.25 × 1.55
TDEE = 2123.9 kcal/day

Standard error ≈ 10% of TDEE
10% of 2123.9 = 212.4 kcal

Band = 2123.9 ± 212.4
Band = 1911.5 to 2336.3 kcal/day

That band is the answer to “how many calories a day” for this person, not 2,124. The midpoint is the best single estimate, but it is not more accurate than the band is wide.

Why the band is not symmetric in practice

The 10% standard error is a summary. The actual error has direction. Mifflin-St Jeor tends to overestimate RMR in people with high body fat and underestimate it in very lean, muscular people, because the equation uses total body mass and does not distinguish fat from lean tissue. Lean tissue is metabolically active; fat is much less so. Two people of the same weight and height can have RMRs that differ by 300 kcal or more if their body compositions differ substantially.

There is also a measurement problem. RMR is measured under strict conditions: fasted, rested, thermoneutral, awake but motionless. A person who calculates their RMR and then eats that amount while moving around is not eating at RMR; they are eating below maintenance. The equation predicts a laboratory condition, not a daily habit.

Maintenance falls as mass falls

A common error is to calculate a calorie target once and hold it for the duration of a diet. Maintenance is not static. As body mass decreases, RMR decreases, because the equation is linear in weight. A 10 kg loss reduces RMR by roughly 100 kcal per day in the Mifflin-St Jeor equation, before any change in activity or adaptive thermogenesis.

Adaptive thermogenesis is the additional reduction in energy expenditure beyond what is predicted by loss of mass. It is real but its magnitude is debated; estimates range from about 50 to 150 kcal per day below predicted in some studies of sustained weight loss, though the measurement is difficult and the effect varies. The direction is consistent: maintenance after weight loss is lower than the equation predicts from the new weight alone.

This means a calorie target that produced loss in month one may produce maintenance in month six. The arithmetic should be repeated at intervals, or the target should be adjusted downward as weight falls.

What people get wrong

The most common mistake is treating the midpoint as a personal requirement. It is a population estimate. The equation was fitted to a sample; the standard error describes how far individual values scatter around the fitted line. A person at the 90th percentile of the error distribution has a true maintenance 10% above the midpoint, and no equation can tell them that from their height and weight alone.

The second mistake is using the activity multiplier as a precise instrument. It is a categorical guess. The gap between adjacent categories is often larger than the difference between two people who describe themselves with the same word.

The third mistake is expecting the scale to move at a constant rate. A 500 kcal daily deficit is often described as producing 0.45 kg per week, based on 7,700 kcal per kilogram of adipose tissue. That figure describes the energy density of adipose tissue, not body weight. Early weight loss includes water and glycogen, which exaggerate the rate. Later, maintenance falls, which reduces the deficit even if intake is unchanged. The 7,700 figure is wrong in both directions at different times.

Using two weeks of data

The estimate is a starting point. The adjustment is empirical. Eat at the midpoint of the band for 14 days, weigh at the same time each morning, and average the readings from the first week and the second week. If the average is stable, the midpoint is close to maintenance. If it is falling by more than about 0.2% of body weight per week, the midpoint is below maintenance. If it is rising, the midpoint is above.

This is not a precise method. Day-to-day weight fluctuates by 1–2 kg with sodium, carbohydrate, and menstrual cycle. The two-week average smooths some of that, but not all. The signal is the trend, not any single reading.

When the arithmetic is not the right tool

Calorie counting is not appropriate for everyone. A history of disordered eating, a clinical condition affecting metabolism or appetite, pregnancy, or medication that alters weight regulation all change the calculation or make the calculation irrelevant. A registered dietitian can provide an estimate that accounts for clinical context. In the United States, the National Eating Disorders Association helpline is available at 1-800-931-2237, and the Academy of Nutrition and Dietetics has a find-a-dietitian tool.

This page is arithmetic. It is not medical advice.