Every calorie calculation — whether for a diet app, a fitness tracker, or a hospital nutrition plan — begins with an estimate of your Basal Metabolic Rate (BMR): the number of calories your body burns at complete rest, just to keep your heart beating, lungs breathing, and cells functioning. Get this number wrong, and every calorie target built on top of it will be wrong too.
The Mifflin-St Jeor equation is the gold standard formula for estimating BMR. It was developed in 1990 by M.D. Mifflin and S.T. St Jeor through careful study of 498 adults, and has since been validated in multiple independent studies as the most accurate predictive equation for resting metabolic rate in healthy adults. This guide explains the formula, how to calculate it step by step, how to convert it to a full daily calorie estimate, and when its limitations apply.
Your Basal Metabolic Rate is the number of calories your body needs to sustain basic physiological functions at complete rest — in a thermoneutral environment, after a full night's sleep, in a fasted state. It accounts for the energy used by your heart, brain, liver, kidneys, lungs, and every other organ simply to keep you alive.
BMR typically accounts for 60–75% of total daily energy expenditure in sedentary adults — meaning that the majority of the calories you burn each day are burned by your body's background processes, not by exercise or activity. This is why crash diets that drastically reduce calories trigger metabolic adaptation: the body downregulates BMR to conserve energy in response to perceived starvation.
The Mifflin-St Jeor equation uses four variables: body weight (in kilograms), height (in centimetres), age (in years), and sex. It produces two slightly different equations for males and females:
For Women:
For Men:
The only difference between the male and female equations is the constant at the end: −161 for women, +5 for men. This 166-calorie difference reflects the average difference in body composition (muscle mass, organ mass) between biological sexes at equivalent weight and height.
Using the female Mifflin-St Jeor formula:
This means this woman burns approximately 1,345 calories per day at complete rest. Her actual daily calorie needs depend on her activity level — see the TDEE section below.
Using the male Mifflin-St Jeor formula:
This man burns approximately 1,768 calories per day at rest, before accounting for any physical activity.
Use our free calorie calculator — powered by the Mifflin-St Jeor equation — to get your personalised BMR and daily calorie target in seconds.
Open Calorie Calculator →BMR alone is not enough to set a calorie target, because it does not account for the calories burned through physical activity. To calculate your Total Daily Energy Expenditure (TDEE) — your actual daily calorie burn — multiply your BMR by an activity factor:
| Activity Level | Description | Multiplier |
|---|---|---|
| Sedentary | Desk job, little or no exercise | BMR × 1.2 |
| Lightly active | Light exercise 1–3 days/week | BMR × 1.375 |
| Moderately active | Moderate exercise 3–5 days/week | BMR × 1.55 |
| Very active | Hard exercise 6–7 days/week | BMR × 1.725 |
| Extra active | Very hard exercise, physical job, or training twice daily | BMR × 1.9 |
Using Example 1 above (woman, BMR = 1,345 calories), if she is lightly active:
TDEE = 1,345 × 1.375 = 1,849 calories/day
This is her maintenance calorie level — the number of calories at which her weight stays stable. To lose weight, she would eat below this number; to gain weight, above it.
No equation can predict your exact metabolic rate — that requires laboratory measurement using indirect calorimetry. What the Mifflin-St Jeor formula provides is the best statistical estimate for the general population. Key accuracy data:
| Formula | Year | Accuracy (within 10%) | Notes |
|---|---|---|---|
| Mifflin-St Jeor | 1990 | 82% | Current gold standard; recommended by the Academy of Nutrition and Dietetics |
| Harris-Benedict (revised) | 1984 | 70% | Older formula; still widely used in medical settings |
| Owen | 1986 | 68% | Simpler formula; less accurate |
| Katch-McArdle | 1975 | Varies | Uses lean body mass; more accurate for athletes if body fat % is known |
| Cunningham | 1980 | Varies | Lean mass-based; best for very lean individuals |
The Katch-McArdle and Cunningham equations can be more accurate than Mifflin-St Jeor for people with known body fat percentages — particularly athletes with very low body fat. However, they require accurate body composition data (typically from DEXA scan or underwater weighing), which most people do not have. For practical use without laboratory measurements, Mifflin-St Jeor remains the best option.
The Mifflin-St Jeor equation accounts for age, sex, weight, and height. But several other factors influence your true metabolic rate that no formula can capture:
Muscle tissue is metabolically more active than fat tissue — it burns approximately 13 calories per kg per day at rest, compared to approximately 4.5 calories per kg per day for fat tissue. Two people of identical weight, height, age, and sex can have meaningfully different BMRs if one has significantly more muscle mass. This is why resistance training raises metabolic rate over time: more muscle means higher BMR.
The thyroid gland regulates metabolic rate through the hormones T3 and T4. Hypothyroidism (underactive thyroid) can reduce BMR by 20–40% below predicted values, while hyperthyroidism can raise it by a similar margin. If your actual calorie needs seem dramatically different from your Mifflin-St Jeor estimate, thyroid function is worth investigating with a doctor.
Studies of identical twins raised in different environments confirm a genetic component to metabolic rate of approximately 40–70%. Some people simply have higher or lower baseline metabolic rates than formula predictions — this is not pathological but represents normal human variation.
Prolonged calorie restriction causes metabolic adaptation — the body reduces its BMR to conserve energy. In people who have completed a significant diet, actual BMR may be 10–20% below what the Mifflin-St Jeor formula predicts. This is one reason why long-term weight maintenance after dieting is more difficult than the initial weight loss: the metabolic rate has been partially suppressed.
Several common medications affect metabolic rate: beta-blockers can reduce BMR by 5–10%, some antidepressants (SSRIs, SNRIs) can reduce it by 5–8%, and corticosteroids can significantly alter both BMR and body composition. If you are on regular medication, discuss its metabolic effects with your prescribing doctor.
Once you have your TDEE, applying it to a weight loss goal is straightforward:
The Mifflin-St Jeor equation was developed in 1990 by Mark D. Mifflin and Sachiko T. St Jeor at the University of Nevada School of Medicine. The researchers recruited 498 healthy subjects (251 women, 247 men) across a range of ages, heights, and weights, measured their resting metabolic rate using indirect calorimetry (the gold standard measurement method), and developed regression equations that best predicted RMR from readily available variables: age, sex, height, and weight.
The 1990 paper, published in the American Journal of Clinical Nutrition, showed that the Mifflin-St Jeor equation outperformed the then-dominant Harris-Benedict equations (developed in 1919) in predicting measured RMR. A landmark 2005 validation study by Frankenfield et al., also published in the Journal of the American Dietetic Association, compared five different prediction equations against measured RMR in 200 subjects and concluded that the Mifflin-St Jeor equation was the most accurate predictor for the general population, correct within 10% of measured RMR for approximately 82% of subjects tested.
This validation established Mifflin-St Jeor as the standard of care in clinical dietetics, where it has remained the recommended equation in the Academy of Nutrition and Dietetics practice guidelines for over two decades. Understanding this history helps explain why the equation is treated with such authority — it is not simply a popular formula but a validated, peer-reviewed tool with a strong evidence base.
Several alternative equations for estimating BMR exist. Understanding how they differ from Mifflin-St Jeor and when each might be preferred helps in choosing the most appropriate formula for different situations.
The original Harris-Benedict equations, developed over a century ago, are still used by many online calculators despite being the least accurate of the major BMR formulas. The 1984 revision by Roza and Shizgal improved accuracy modestly. Studies consistently show Harris-Benedict overestimates BMR by 5–8% on average in modern populations — likely because the original 1919 subjects were leaner and more physically active than contemporary populations, skewing the equation's baseline assumptions. For most people, Harris-Benedict produces a higher calorie target than Mifflin-St Jeor, which can be misleading for weight management purposes.
The Katch-McArdle equation calculates BMR from lean body mass (LBM) rather than total body weight: BMR = 370 + (21.6 × LBM in kg). This approach is theoretically superior for individuals with significantly above- or below-average body fat percentages, because metabolic rate is driven primarily by lean tissue (muscle, organs) rather than fat tissue. A highly muscular woman and a sedentary woman of the same age, height, and weight will have different BMRs — something Mifflin-St Jeor cannot capture but Katch-McArdle can, provided lean body mass is accurately measured. The practical limitation is that accurate LBM measurement requires a DEXA scan or hydrostatic weighing; consumer body fat scales are insufficiently accurate for this purpose.
Like Katch-McArdle, the Cunningham equation uses lean body mass as its primary input and is particularly well-suited to athletic populations. It tends to produce slightly higher BMR estimates than Katch-McArdle and is preferred by some sports dietitians for elite athletes. For the general population without accurate body composition data, it offers no practical advantage over Mifflin-St Jeor.
BMR represents only your resting energy expenditure. To estimate your Total Daily Energy Expenditure (TDEE) — the actual number of calories you burn across an entire day — your BMR must be multiplied by an activity factor that accounts for your movement level. The standard activity multipliers used with Mifflin-St Jeor are:
Research consistently shows that most people overestimate their activity level when selecting a multiplier. A 2011 study in PLOS ONE found that self-reported physical activity overestimates actual activity by approximately 50% in sedentary and lightly active individuals. This overestimation leads to inflated TDEE estimates and consequently inadequate calorie deficits for those trying to lose weight. Starting with a lower activity multiplier and adjusting based on real-world results (tracking weight change over 2–3 weeks) is more reliable than selecting the highest plausible category upfront.
Once you have calculated your TDEE using the Mifflin-St Jeor equation and an appropriate activity multiplier, the practical application depends on your goals.
Create a calorie deficit by eating below your TDEE. The evidence supports a deficit of 300–500 calories per day for most people, producing a weight loss rate of approximately 0.3–0.5 kg per week. Larger deficits (500–750 cal/day) can produce faster short-term weight loss but increase the risk of muscle loss, fatigue, hormonal disruption (particularly in women), and rebound weight gain after the diet ends. For women specifically, deficits below 1,200 calories per day are generally not recommended without medical supervision, as intakes this low make adequate nutrition extremely difficult to achieve.
Eat at your calculated TDEE. In practice, this means tracking food intake against your target, which research shows improves weight maintenance outcomes significantly. A 2019 study in Obesity found that self-monitoring of food intake was the single strongest predictor of successful weight maintenance at one and two years post-weight-loss, more predictive than exercise frequency, diet quality, or initial weight loss magnitude.
Eat above your TDEE — typically 200–300 calories above for women. This calorie surplus, combined with progressive resistance training, provides the energy and amino acid availability needed for muscle protein synthesis. The rate of muscle gain for women is significantly slower than for men due to lower testosterone levels: realistic expectations are 0.5–1 kg of muscle per month in the early stages of training, slowing significantly as training experience increases.
One of the most important — and most frequently overlooked — aspects of long-term calorie management is metabolic adaptation: the reduction in metabolic rate that occurs during sustained calorie restriction. When you eat below your TDEE consistently, your body adapts by reducing BMR, decreasing non-exercise activity thermogenesis (NEAT), and improving metabolic efficiency — all of which reduce total calorie expenditure and gradually shrink your actual deficit.
This metabolic adaptation is why weight loss typically slows after 8–12 weeks even when calorie intake remains constant — and why the Mifflin-St Jeor result should be periodically recalculated as body weight changes. For every kilogram of weight lost, BMR decreases by approximately 13–15 calories per day. A woman who loses 10 kg will have a BMR approximately 130–150 calories lower than predicted by her original calculation, requiring an adjustment to her calorie target to continue making progress.
Practical strategies for managing metabolic adaptation include: diet breaks (returning to TDEE for 1–2 weeks after 8–12 weeks of deficit, which partially restores metabolic rate); refeeds (planned periods of higher carbohydrate intake that temporarily restore leptin levels and metabolic rate); and periodically recalculating TDEE based on current weight rather than starting weight.
Several systematic errors reduce the accuracy of Mifflin-St Jeor calculations in practice. Avoiding these mistakes significantly improves the reliability of the calorie targets derived from the equation.
Using current weight rather than a stable recent average: Body weight fluctuates by 1–3 kg day to day due to hydration, food volume in the digestive tract, and hormonal changes (particularly in women across the menstrual cycle). Using a single morning weight for the calculation can introduce meaningful error. Use an average of daily weights across 7–10 days for the most stable input.
Selecting too high an activity multiplier: As noted above, most people overestimate their activity level. When uncertain between two categories, select the lower one and adjust based on real-world weight change data over 2–3 weeks. This prevents the common scenario of calculating a deficit that is insufficient for weight loss because the starting calorie target was overestimated.
Not recalculating as weight changes: TDEE changes as body weight changes. A calorie target calculated at 80 kg is not appropriate at 70 kg. Recalculate your BMR and TDEE at each 5 kg of weight change, or every 8–12 weeks — whichever comes first.
Treating the result as exact rather than approximate: The Mifflin-St Jeor equation predicts measured BMR within 10% for approximately 82% of people — meaning for approximately 18% of people, the prediction is less accurate. Individual variation in metabolic rate is real and meaningful. If your weight is not changing as expected based on your calculated deficit after 3–4 weeks of consistent tracking, adjust your calorie target based on observed results rather than assuming the calculation is correct and your tracking is wrong.
Ignoring the calorie content of alcohol: Alcohol provides 7 calories per gram — more than protein or carbohydrates (4 cal/g each) and approaching fat (9 cal/g). Regular alcohol consumption represents a significant hidden calorie source that the Mifflin-St Jeor calculation does not account for. A glass of wine (120 cal), a pint of beer (200 cal), or a cocktail (200–300 cal) can meaningfully reduce the effective calorie deficit without appearing in food tracking if beverages are not logged.