What Is a Good BMR Number? Mifflin-St Jeor vs Harris-Benedict
Quick Take
Mifflin-St Jeor is the most accurate BMR formula available today. Published in 1990, it outperforms the older Harris-Benedict equation from 1918 (revised 1984). For most adults, BMR ranges from about 1,300 to 2,000 kcal/day depending on age, sex, and body size. Your BMR represents the minimum calories your body needs to sustain vital functions at rest.
Ever wondered how many calories your body burns just by breathing, sleeping, and existing? That is your BMR — Basal Metabolic Rate. It is the foundation of every calorie calculator, TDEE estimate, and nutrition plan. Yet most people have never heard of BMR, let alone know how to calculate it or what a healthy range looks like.
Let me cut to the chase: not all BMR formulas are created equal. The Mifflin-St Jeor equation from 1990 is the one nutrition experts actually trust. The old Harris-Benedict formula from 1918? It still appears in many calculators, but it was based on a tiny sample of people over a century ago. Times change, and so do our bodies.
Curious about your own BMR? Use the BMR Calculator which implements Mifflin-St Jeor, Harris-Benedict revised, and Katch-McArdle formulas side by side. For total daily energy expenditure calculations, visit the TDEE Calculator which applies activity factor multipliers to your BMR.
BMR vs RMR: What Is the Difference?
BMR and RMR are frequently used interchangeably in popular writing, but they refer to measurements made under technically different laboratory conditions. Understanding the distinction matters when interpreting published validation studies and comparing formula accuracy.
BMR (Basal Metabolic Rate)
BMR is measured under the most restrictive conditions: a thermoneutral environment (approximately 75–81°F / 24–27°C), 10 to 12 hours of overnight fasting, no vigorous physical activity in the preceding 12 to 24 hours, no stimulants, the subject lying motionless in a supine position, and either asleep or immediately post-awakening. BMR represents the absolute minimum energy expenditure required to sustain vital organ function under these strictly controlled conditions.
RMR (Resting Metabolic Rate)
RMR is measured under less restrictive conditions: typically 3 to 4 hours post-prandial (after a meal), no vigorous exercise in the preceding few hours, and the subject resting quietly. Because of the thermic effect of recent food intake and the less strict fasting requirement, measured RMR values are typically 5% to 15% higher than measured BMR values for the same individual. In everyday usage, however, the two terms are often used interchangeably to refer to the output of any prediction equation.
Mifflin-St Jeor 1990: Today's De Facto Standard
The Mifflin-St Jeor (MSJ) equations were published in 1990 by Mark D. Mifflin, Sachiko T. St Jeor, and colleagues in the American Journal of Clinical Nutrition. The study used 498 indirect calorimetry measurements carefully selected to represent a broader range of age, weight, and BMI values than prior datasets. The Academy of Nutrition and Dietetics has identified Mifflin-St Jeor as the most accurate population BMR prediction equation for clinical and public health use.
Mifflin-St Jeor Equations
For males: BMR (kcal/day) = 10 × weight(kg) + 6.25 × height(cm) − 5 × age(years) + 5
For females: BMR (kcal/day) = 10 × weight(kg) + 6.25 × height(cm) − 5 × age(years) − 161
Notice that the weight, height, and age coefficients are identical between the sexes — only the constant term differs (+5 for males, −161 for females), a difference of 166 kcal/day. This structural simplicity is one of the practical advantages of the MSJ formulation and contributes to its consistent performance across diverse populations.
Harris-Benedict Formula: Historical Context
The Harris-Benedict equations are the oldest continuously used BMR prediction formulas in modern nutrition science. They were published in 1918 and 1919 by James Arthur Harris and Francis Gano Benedict, based on indirect calorimetry measurements of 239 adult subjects (136 men, 103 women), primarily of European ancestry residing in the Northeastern United States.
Original 1918 Harris-Benedict Equations
For males: BMR = 66.4730 + 13.7516 × weight(kg) + 5.0033 × height(cm) − 6.7550 × age(years)
For females: BMR = 655.0955 + 9.5634 × weight(kg) + 1.8496 × height(cm) − 4.6756 × age(years)
The equations were remarkably successful for their era and remained in widespread clinical use for more than six decades. However, the narrow demographic range of the original sample became a limitation when applied to more diverse modern populations.
1984 Roza-Shizgal Revision
In 1984, D. Roza and H. M. Shizgal published a re-estimation of the Harris-Benedict coefficients using a larger and more demographically diverse dataset. The revised equations reduced systematic overprediction observed in certain demographic subgroups and remain in use today as the "Harris-Benedict revised" or "Roza-Shizgal" version.
Revised male: BMR = 88.362 + 13.397 × weight(kg) + 4.799 × height(cm) − 5.677 × age(years)
Revised female: BMR = 447.593 + 9.247 × weight(kg) + 3.098 × height(cm) − 4.330 × age(years)
Normal BMR Ranges by Age
What constitutes a "normal" BMR depends on several factors, with age and sex being the primary determinants. As we age, BMR naturally declines — typically by about 1–2% per year after age 30, due primarily to loss of lean muscle mass. The following table provides approximate BMR ranges for US adults at different ages, calculated using the Mifflin-St Jeor equation for reference body sizes (based on NHANES median heights and weights).
| Age Group | Males (kcal/day) | Females (kcal/day) |
|---|---|---|
| 18 – 24 | 1,700 – 2,000 | 1,350 – 1,600 |
| 25 – 34 | 1,650 – 1,950 | 1,300 – 1,550 |
| 35 – 44 | 1,550 – 1,850 | 1,250 – 1,500 |
| 45 – 54 | 1,500 – 1,750 | 1,200 – 1,450 |
| 55 – 64 | 1,400 – 1,700 | 1,150 – 1,400 |
| 65+ | 1,300 – 1,600 | 1,100 – 1,350 |
These ranges are approximate and represent average values. Your actual BMR may differ by ±200–300 kcal/day or more depending on body composition, thyroid function, genetics, and activity level. Use the BMR Calculator to estimate your personal BMR using the Mifflin-St Jeor equation.
Age and Gender Differences in BMR
BMR declines with age for both sexes, but the rate and magnitude vary. After age 30, BMR typically decreases by about 0.5–1% per year. The primary driver is sarcopenia — the gradual loss of lean muscle mass that begins around age 30 and accelerates after 50. Since muscle tissue is metabolically active and consumes more energy at rest than adipose tissue, losing muscle means burning fewer calories at rest.
Men generally have higher BMRs than women at the same age because they naturally carry more lean muscle mass. The sex difference is most pronounced during young adulthood (ages 18–30) and gradually narrows with age as muscle mass declines in both sexes.
Body composition is the single most important predictor of BMR after accounting for age and sex. A 200-pound man with 20% body fat will have a significantly higher BMR than a 200-pound man with 40% body fat, even though they weigh the same. This is why the Katch-McArdle formula, which uses lean body mass directly, can provide more personalized estimates for individuals with known body composition data.
Factors Affecting BMR Beyond Age and Sex
Several factors can influence your resting metabolic rate beyond the basic variables used in prediction equations:
Body Composition
Lean mass is the primary driver of BMR. Skeletal muscle, heart muscle, liver, kidneys, and brain are the major energy-consuming organs at rest. Individuals with greater lean mass have higher BMRs, while those with more adipose tissue have lower BMRs per unit of body weight.
Thyroid Function
The thyroid gland produces hormones that regulate metabolic rate. Hyperthyroidism (overactive thyroid) can increase BMR by 20–30%, while hypothyroidism (underactive thyroid) can reduce it by 20–40%. These effects are not captured by any prediction equation and require clinical assessment.
Physical Activity History
Chronic endurance exercise training can increase BMR by 5–10%, even at rest. This is partly due to increased lean mass and partly due to higher mitochondrial density in muscle tissue. Resistance training similarly increases BMR through lean mass gains.
Recent Weight History
Significant weight loss or gain can temporarily alter BMR. Rapid weight loss often results in a disproportionate loss of lean mass, which can lower BMR beyond what would be predicted from the new weight alone — a phenomenon sometimes discussed as "adaptive thermogenesis."
Genetics
Heritability studies suggest that 20–30% of the variation in BMR between individuals is genetic. This means two people with the same age, sex, weight, height, and body composition can still have measurably different resting metabolic rates.
Formula Accuracy Comparison
Published validation studies consistently rank Mifflin-St Jeor as the most accurate BMR prediction equation for use in clinical and public health settings. The following table summarizes approximate accuracy metrics from published validation research:
| Prediction Equation | R² Range | RMSE (kcal/day) | Expert Ranking |
|---|---|---|---|
| Mifflin-St Jeor (1990) | 0.70 – 0.85 | 100 – 200 | Most accurate |
| Harris-Benedict Revised (1984) | 0.60 – 0.80 | 130 – 230 | Secondary |
| Harris-Benedict Original (1918) | 0.55 – 0.75 | 150 – 280 | Historical only |
| Katch-McArdle (LBM-based) | 0.65 – 0.82 | 120 – 220 | Conditional |
R² values represent the proportion of variance in measured indirect calorimetry values explained by each equation. Higher R² indicates better prediction. RMSE (root mean squared error) indicates the typical prediction error in kilocalories per day. For any individual, the actual measured BMR can differ from the predicted value by more than these population-average error ranges.
Worked Examples
Example 1: 30-Year-Old Female, 154 lb, 5'9"
Weight: 154 lb = 70 kg. Height: 5'9" = 175 cm.
Mifflin-St Jeor: (10 × 70) + (6.25 × 175) − (5 × 30) − 161 = 700 + 1093.75 − 150 − 161 = 1,482.75 kcal/day
Harris-Benedict Revised: 447.593 + (9.247 × 70) + (3.098 × 175) − (4.330 × 30) = 447.593 + 647.29 + 542.15 − 129.9 = 1,507.13 kcal/day
Example 2: 25-Year-Old Male, 187 lb, 5'11"
Weight: 187 lb = 85 kg. Height: 5'11" = 180 cm.
Mifflin-St Jeor: (10 × 85) + (6.25 × 180) − (5 × 25) + 5 = 850 + 1125 − 125 + 5 = 1,855 kcal/day
Harris-Benedict Revised: 88.362 + (13.397 × 85) + (4.799 × 180) − (5.677 × 25) = 88.362 + 1138.745 + 863.82 − 141.925 = 1,949.00 kcal/day
Notice that Harris-Benedict revised tends to produce slightly higher estimates than Mifflin-St Jeor for these examples. This pattern is consistent across most validation studies, though the difference is usually modest (5–10%) for individuals within normal BMI ranges.
Known Limitations
All BMR prediction equations are population regression models. The coefficients were optimized to minimize prediction error across a training sample, which means accuracy decreases at the extremes of the BMI distribution. For individuals with BMI below 16 or above 40, prediction error increases and the direction of bias can vary.
Additional limitations include: (1) the input variables capture only some of the known factors contributing to BMR variation — genetic factors, thyroid function, and body composition beyond lean vs fat ratio are not captured in weight-height-age-sex inputs; (2) the equations were trained primarily on European and US populations, and some non-Western groups may show systematic average differences; (3) they provide point estimates, and any individual's measured BMR can differ from the predicted value by more than the population RMSE.