BMR & TDEE Calculator — Mifflin-St Jeor Equation

About

Basal Metabolic Rate (BMR) is the minimum amount of energy required to maintain vital functions at complete rest in a thermoneutral environment, including breathing, circulation, cellular metabolism, and organ function. It accounts for approximately 60–75% of Total Daily Energy Expenditure (TDEE) in sedentary individuals. The concept was first studied by Harris and Benedict in 1919, who developed the first widely used predictive equation based on 136 healthy subjects. The Mifflin-St Jeor equation, published in 1990 and based on 498 healthy individuals (251 men, 247 women), is now considered the most accurate predictive equation for estimating resting energy expenditure in the general population, being accurate within ±10% in 70–80% of individuals. TDEE is calculated by multiplying BMR by an activity factor that accounts for physical activity, the thermic effect of food (approximately 10% of caloric intake), and non-exercise activity thermogenesis (NEAT). Daily energy requirements vary significantly between individuals based on lean body mass, hormonal status, genetic factors, and environmental conditions. The Academy of Nutrition and Dietetics and the American Society for Parenteral and Enteral Nutrition (ASPEN) recommend the Mifflin-St Jeor equation as the preferred predictive equation for clinical use. Evidence level: Grade B, supported by multiple validation studies across diverse populations.

Formula

Male BMR = 10W + 6.25H - 5A + 5 | Female BMR = 10W + 6.25H - 5A - 161 | TDEE = BMR × Activity Factor

The Mifflin-St Jeor equation uses weight (W in kg), height (H in cm), and age (A in years). For males: BMR = 10W + 6.25H − 5A + 5. For females: BMR = 10W + 6.25H − 5A − 161. Each component has a physiological basis: the weight coefficient (10) reflects the energy cost of maintaining adipose and lean tissue; the height coefficient (6.25) accounts for the higher metabolic rate of taller individuals due to greater organ mass; the age coefficient (−5) reflects the decline in BMR of approximately 1–2% per decade after age 20 due to loss of lean body mass. The sex adjustment (−161 for women, +5 for men) accounts for the higher average lean mass and metabolic rate in males. For example, a 30-year-old woman weighing 65 kg, 165 cm tall: BMR = 10(65) + 6.25(165) − 5(30) − 161 = 650 + 1031.25 − 150 − 161 = 1370 kcal/day. TDEE is BMR multiplied by an activity factor: Sedentary (1.2, little or no exercise), Lightly Active (1.375, 1–3 days/week), Moderately Active (1.55, 3–5 days/week), Active (1.725, 6–7 days/week), and Very Active (1.9, intense daily exercise plus physical job). To interpret BMR, compare it to age- and sex-matched population norms. A low BMR relative to predicted may indicate hypothyroidism or metabolic adaptation from caloric restriction. A high BMR may indicate hyperthyroidism, fever, or increased sympathetic activity. TDEE is the actual number used for weight management: consuming fewer calories than TDEE produces weight loss, while consuming more produces weight gain. A deficit of 500–1000 kcal/day below TDEE typically produces 0.5–1 kg of weight loss per week.

Score Interpretation

The Mifflin-St Jeor equation is the recommended predictive equation for estimating resting energy expenditure by the Academy of Nutrition and Dietetics and ASPEN. It has largely replaced the older Harris-Benedict equation, which tends to overestimate BMR by 5–15%, particularly in overweight and obese individuals. In clinical practice, BMR and TDEE estimates guide several critical interventions. In obesity management, the American College of Cardiology/AHA guidelines recommend creating a calorie deficit of 500–750 kcal/day from TDEE for weight loss. In critical care, ASPEN/SCCM guidelines recommend calculating energy requirements using predictive equations or indirect calorimetry for mechanically ventilated patients — overfeeding can cause hyperglycemia, hypercapnia, and hepatic steatosis, while underfeeding leads to malnutrition and impaired immune function. In thyroid disorders, BMR correlates with thyroid hormone levels: hyperthyroidism increases BMR by 20–80%, while hypothyroidism decreases it by 15–40%. Serial BMR estimation helps monitor response to antithyroid therapy or levothyroxine replacement. In sports nutrition, TDEE estimation is essential for athletes to optimize performance — endurance athletes may require 3000–5000 kcal/day during peak training. BMR estimates also have applications in determining disability and metabolic health assessments. However, clinicians should recognize that predictive equations have a ±20% accuracy at the individual level. Indirect calorimetry, when available, provides more precise measurements, especially for patients at extremes of body weight, critical illness, or those not responding to estimated caloric targets. The Mifflin-St Jeor equation is also used in the diagnosis of metabolic syndrome, where BMR-adjusted metabolic rate assessments can help identify patients with metabolic inflexibility.

Reference Ranges