For educational and tracking purposes only. Results are estimates and are not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare professional.
Calculator tool
How this calculator works
Use the explanation to understand the formula, assumptions, and practical limits behind the calculator result.
What Is BMR?
Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest to keep vital functions running — breathing, circulation, cell production, and temperature regulation. It accounts for 60–75% of total daily energy expenditure for most people and is the baseline every calorie target is built on top of.
The Formulas
Where = weight in kg, = height in cm, = age in years.
Mifflin-St Jeor (recommended):
Men:
Women:
Revised Harris-Benedict (classic):
Men:
Women:
Mifflin-St Jeor (published 1990) consistently outperforms Harris-Benedict in validation studies — mean error ~10% versus ~15%. Both carry a standard deviation of roughly ±200 kcal, which means the formula gives you a useful estimate, not a precise measurement.
Worked Example
Inputs: Male, age 30, weight 75 kg, height 175 cm
Mifflin-St Jeor: kcal/day
Harris-Benedict: kcal/day
The two formulas differ by about 64 kcal — a typical gap. Use Mifflin-St Jeor unless your clinical context specifically requires Harris-Benedict.
From BMR to Daily Calorie Needs
To estimate Total Daily Energy Expenditure (TDEE), multiply BMR by an activity factor:
| Activity Level | Multiplier |
|---|---|
| Sedentary (desk job, 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 + daily training) | ×1.9 |
For the example above (BMR = 1,699 kcal): sedentary TDEE = kcal; moderately active = kcal.
A daily deficit of 500 kcal below TDEE produces approximately 0.45 kg (1 lb) of fat loss per week based on the ~7,700 kcal per kilogram of fat approximation.
How Age and Sex Affect BMR
BMR falls approximately 1–2% per decade after age 20, driven primarily by muscle mass loss (sarcopenia). Muscle tissue burns about 6 kcal/kg/day at rest; fat tissue burns only 2 kcal/kg/day. Resistance training that preserves muscle partially offsets this decline — an effect the formula does not capture.
The sex offset (+5 for men, −161 for women in Mifflin-St Jeor) reflects the average lean mass difference between sexes at the same height and weight.
Common Mistakes
Eating at BMR. BMR is the minimum at complete rest. Eating at BMR while active creates a large deficit and risks lean mass loss. Use TDEE — BMR multiplied by your activity level — as your actual calorie baseline.
Treating the result as exact. Both formulas carry a ±200 kcal standard deviation. Track actual intake and body weight trend for 2–3 weeks before deciding the formula is off.
Using the formula with medical conditions. Thyroid disorders, cachexia, significant obesity, and several medications alter metabolic rate in ways the formulas do not model. Clinical settings use indirect calorimetry (measured oxygen consumption) for accurate individual estimates.
Frequently asked questions
What is the difference between BMR and TDEE?
BMR is the calories burned at complete rest — essentially what your body needs if you lay in bed all day. TDEE (Total Daily Energy Expenditure) adds calories burned from physical activity, the thermic effect of food (~10% of calories consumed), and non-exercise activity thermogenesis (NEAT — fidgeting, standing, casual walking).
For a sedentary person, TDEE ≈ BMR × 1.2; for a very active person, TDEE can reach BMR × 1.9 or higher. For weight management, TDEE is always the more relevant number — not BMR.
Which BMR formula is more accurate — Mifflin-St Jeor or Harris-Benedict?
Mifflin-St Jeor (1990) is generally more accurate for modern populations, with a mean error of about 10% in validation studies. Harris-Benedict (1919, revised 1984) consistently overpredicts BMR by 5–15% in non-obese adults.
Both formulas have roughly ±200 kcal standard deviation — meaning 68% of people fall within 200 kcal of the predicted value. The choice between formulas matters less than the activity multiplier you apply to convert BMR into TDEE.
How does age affect BMR?
BMR decreases approximately 1–2% per decade after age 20, primarily because muscle mass naturally declines (sarcopenia) while fat mass tends to increase. Muscle tissue burns about 6 kcal/kg/day at rest, while fat burns only 2 kcal/kg/day.
Hormonal changes — declining growth hormone, sex hormones, and thyroid output — contribute additional reductions beyond muscle loss alone. Resistance training that preserves or builds muscle mass is the most effective behavioral intervention to slow BMR decline with age.
Why does crash dieting reduce BMR?
Severe caloric restriction can trigger metabolic adaptation: the body reduces energy expenditure, and lean mass may be lost if protein intake, resistance training, and total calories are too low.
The size of the drop varies widely by person, diet duration, starting body composition, and activity. A moderate deficit is generally easier to sustain than an extreme restriction, but the right target depends on health status and goals.
If you have a medical condition, a history of disordered eating, or large weight-loss goals, use BMR as a planning estimate and work with a qualified health professional.
