Medical AIMifflin-St Jeor BMR and TDEE calculator
The Mifflin-St Jeor equation estimates resting energy expenditure (the calories burned at rest) from weight, height, age, and sex. Multiplying by an activity factor gives total daily energy expenditure, a starting point for calorie planning in healthy adults.
How to measure each input
- Weight and height
- Use current measured weight in kilograms and standing height in centimetres. Self-reported figures introduce error, so measure where possible.
- Sex constant
- The equation uses +5 for men and -161 for women, reflecting the lean-mass difference observed in the derivation cohort.
- Activity factor
- Pick the activity multiplier that best matches habitual weekly activity. The factors are conventional values applied to resting expenditure, not part of the original regression.
Interpretation
| Band | Meaning |
|---|---|
| Implausible result | A non-positive BMR signals an out-of-range input (for example a very low weight or very high age). Recheck the entries; the equation is only validated for healthy adults. |
| Estimated resting metabolic rate | This is the estimated number of calories burned at rest over 24 hours. Total daily energy expenditure (shown alongside) adds the activity factor. Use these as planning estimates, not measured values: individual rates vary by roughly 10% even in healthy adults. |
Pitfalls, exclusions and caveats
- Derived and validated in healthy adults (264 normal-weight and 234 obese subjects). It is not validated at the extremes of body composition, in pregnancy, in children, or in critical illness.
- BMR is the resting value; the TDEE figure depends entirely on the chosen activity factor, which is a coarse estimate. Two people with the same BMR can differ widely in real daily expenditure.
- Predicts measured resting expenditure within about 10% in most people, so treat the output as an approximation and adjust to observed weight trends.
- Acute illness, fever, large amounts of lean mass, and some medications shift energy needs in ways the equation does not capture.
BMR (kcal/day) = 10 x weight(kg) + 6.25 x height(cm) - 5 x age(years) + sex constant, where the sex constant is +5 for men and -161 for women. TDEE = BMR x activity factor (sedentary 1.2, light 1.375, moderate 1.55, very active 1.725, extra active 1.9).The Mifflin-St Jeor equation was published by Mifflin et al. (1990). This implementation is an educational tool and is not affiliated with the original authors.
Frequently asked
What is the difference between BMR and TDEE?
Basal (resting) metabolic rate is the calories your body uses at rest over 24 hours. Total daily energy expenditure multiplies that by an activity factor to estimate the calories you actually use across a typical day, including movement and exercise.
Why does the formula use +5 for men and -161 for women?
Those sex-specific constants came out of the original regression on 498 healthy adults and approximate the difference in lean body mass between men and women at the same weight, height, and age.
Can I use this in pregnancy or critical illness?
No. The equation was derived in healthy non-pregnant adults. Pregnancy, critical illness, and extremes of body composition change energy needs in ways it does not model, so use validated condition-specific methods or indirect calorimetry instead.
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