Mifflin-St Jeor Equation (Female)
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Variables
| Symbol | Name | Unit | Description |
|---|---|---|---|
| $BMR$ | Basal Metabolic Rate | kcal/day | Calories burned at complete rest over 24 hours. |
| $w$ | Body weight | kg | Total body mass in kilograms. |
| $h$ | Height | cm | Standing height in centimetres. |
| $a$ | Age | years | Age in whole years. |
Mifflin-St Jeor Equation for Females
The female version of the Mifflin-St Jeor equation estimates BMR as:
$$BMR = 10w + 6.25h - 5a - 161$$
The only difference from the male equation is the constant term: −161 instead of +5. This 166 kcal/day difference reflects the average metabolic gap between men and women of identical weight, height, and age, primarily driven by differences in lean muscle mass and hormonal profiles.
Why Women Have a Lower BMR
Biologically, females tend to carry a higher percentage of body fat and less skeletal muscle than males of comparable body weight. Since metabolic rate is largely determined by lean tissue (muscle, organ mass, bone), this difference leads to a systematically lower BMR. The sex-specific constant captures this population-average difference empirically.
Practical Use
To use the equation correctly:
- Convert weight to kilograms (1 lb = 0.4536 kg).
- Convert height to centimetres (1 inch = 2.54 cm).
- Multiply each variable by its coefficient and sum.
- Multiply the resulting BMR by your activity factor to obtain TDEE.
Activity Multipliers (for TDEE)
| Activity Level | Multiplier | Description |
|---|---|---|
| Sedentary | 1.2 | Desk job, little exercise |
| Lightly active | 1.375 | Light exercise 1–3 days/week |
| Moderately active | 1.55 | Moderate exercise 3–5 days/week |
| Very active | 1.725 | Hard exercise 6–7 days/week |
| Extremely active | 1.9 | Physical job + hard exercise |
Menstrual Cycle and BMR
Research indicates that BMR fluctuates by approximately 2–10% across the menstrual cycle. It is lowest in the early follicular phase and peaks in the luteal phase (after ovulation), rising by roughly 100–300 kcal/day. The Mifflin-St Jeor equation uses a single average value and does not capture this intra-cycle variation.
Derivation & History
The female constant of −161 was determined by the same 1990 Mifflin and St Jeor regression study that produced the male equation. After controlling for weight, height, and age, the regression model found that being female was associated with a BMR approximately 166 kcal/day lower than being male. The male constant (+5) and female constant (−161) differ by 166 kcal/day, representing the population-average sex difference in resting energy expenditure after accounting for anthropometric variables.
The figure aligns well with doubly labelled water studies, which independently measured total energy expenditure and found sex differences of similar magnitude in age- and weight-matched subjects.
Worked Examples
28-year-old female, lightly active
- 10 × 62 = 620
- 6.25 × 165 = 1,031.25
- 5 × 28 = 140
- BMR = 620 + 1,031.25 − 140 − 161 = 1,350.25 kcal/day
- TDEE = 1,350 × 1.375 (lightly active) ≈ 1,857 kcal/day
Result: BMR ≈ 1,350 kcal/day; TDEE ≈ 1,857 kcal/day
50-year-old female, sedentary
- 10 × 70 = 700
- 6.25 × 160 = 1,000
- 5 × 50 = 250
- BMR = 700 + 1,000 − 250 − 161 = 1,289 kcal/day
Result: BMR ≈ 1,289 kcal/day
Edge Cases & Limitations
Menopause: Estrogen decline can decrease BMR by up to 5%; the equation does not account for hormonal status.
Pregnancy and lactation: Caloric needs increase substantially (roughly +300–500 kcal/day); use pregnancy-specific guidelines instead.
Polycystic ovary syndrome (PCOS): Some studies report lower BMR in PCOS patients relative to predictions, possibly due to insulin resistance.
Post-bariatric surgery: Metabolic adaptation can lower actual BMR well below the predicted value; close clinical monitoring is required.
Real-World Applications
Dietitians and GPs use this equation to set evidence-based calorie targets for female patients managing weight, eating disorders, or chronic conditions. Fertility clinics assess energy adequacy using BMR calculations to identify hypocaloric intake that may suppress ovulation. Female athletes use it to avoid Relative Energy Deficiency in Sport (RED-S), a syndrome caused by chronic under-fuelling. Calorie-counting apps apply this formula as the default BMR model for female users.