BMR Calculator (Mifflin-St Jeor)
Your Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest to maintain vital functions — breathing, circulation, and cell repair. The Mifflin-St Jeor equation (1990) is consistently rated the most accurate predictive formula for healthy adults in clinical literature. Enter your sex, weight, height, and age to get your BMR and Total Daily Energy Expenditure (TDEE) based on how active you are.
When to use this calculator
- Setting a science-backed calorie target for weight loss or muscle gain
- Estimating how many calories to eat on rest days vs. training days
- Comparing BMR changes after significant weight loss or gain
- Supporting a dietitian or nutritionist consultation with baseline data
- Tracking metabolic changes with age over multiple years
How it works
2 min readWhat is Basal Metabolic Rate (BMR)?
Basal Metabolic Rate is the number of calories your body burns at rest to maintain essential functions like breathing, circulation, and cell repair. The Mifflin-St Jeor equation, developed in 1990, is the most accurate formula for calculating BMR in healthy adults. It accounts for age, weight, height, and sex to estimate daily calorie requirements.
How It Works
The Mifflin-St Jeor equation was developed by M.D. Mifflin and S.T. St Jeor in 1990 after studying 498 healthy adults. A 2005 meta-analysis by the Academy of Nutrition and Dietetics found it predicted resting energy expenditure within 10% of measured values in roughly 82% of non-obese subjects — outperforming the older Harris-Benedict equation.
Formula
// Male
BMR = (10 × weight_kg) + (6.25 × height_cm) − (5 × age) + 5
// Female
BMR = (10 × weight_kg) + (6.25 × height_cm) − (5 × age) − 161
// TDEE
TDEE = BMR × activity_factorUnit conversions applied before the formula:
Activity Multipliers (Harris-Benedict scale)
| Level | Multiplier | Description |
|---|---|---|
| Sedentary | 1.200 | Desk job, no structured exercise |
| Lightly active | 1.375 | Light cardio or weights 1–3 days/week |
| Moderately active | 1.550 | Moderate workouts 3–5 days/week |
| Very active | 1.725 | Hard training 6–7 days/week |
| Extra active | 1.900 | Twice-daily training or labor-intensive job |
Worked Example
Female, 154 lb (69.9 kg), 5′5″ (165.1 cm), age 35, sedentary:
BMR = (10 × 69.9) + (6.25 × 165.1) − (5 × 35) − 161
= 699 + 1031.9 − 175 − 161
= 1,394.9 ≈ 1,395 kcal/day
TDEE = 1,395 × 1.2 = 1,674 kcal/dayWhen NOT to Apply / Limitations
Frequently asked questions
What is the difference between BMR and TDEE?
BMR (Basal Metabolic Rate) is the calories your body needs at complete rest — no movement, no digestion. TDEE (Total Daily Energy Expenditure) adds the calories burned through all daily activity on top of BMR. Eating at your TDEE maintains your current weight; eating below it creates a deficit for weight loss.
Why is Mifflin-St Jeor considered more accurate than Harris-Benedict?
Harris-Benedict was developed in 1919 on a small sample. Mifflin-St Jeor (1990) used a larger, more diverse modern cohort. A 2005 Academy of Nutrition and Dietetics review found Mifflin-St Jeor predicted measured REE within 10% for about 82% of subjects vs. roughly 68% for Harris-Benedict.
How do I choose the right activity level?
Be honest and conservative. Most people overestimate their activity. If you work a desk job and walk 20–30 minutes most days, choose 'Lightly active.' Structured workouts 4–5 days/week at moderate intensity = 'Moderately active.' Elite athletes or people with physically demanding jobs may qualify for 'Extra active.'
Can I use this calculator to set a weight-loss calorie goal?
Yes. A common starting point is TDEE minus 500 kcal/day, which produces roughly 1 lb (0.45 kg) of fat loss per week. Avoid dropping below your BMR without medical supervision, as this can reduce muscle mass and metabolic rate over time.
Does BMR decrease with age?
Yes. The Mifflin-St Jeor formula includes age as a negative factor (−5 × age). A 60-year-old man has a BMR approximately 125 kcal/day lower than a 35-year-old with identical weight and height. Muscle mass loss (sarcopenia) with age also contributes to lower metabolic rates.
Is this calculator accurate for athletes with high muscle mass?
The Mifflin-St Jeor equation does not account for body composition directly. Highly muscular individuals may have a higher actual BMR than the formula predicts, because lean muscle burns more calories at rest than fat. For athletes, the Katch-McArdle formula (which uses lean body mass) may be more precise.
What units does this calculator accept?
You can enter weight in pounds (lb) or kilograms (kg), and height in inches (in) or centimeters (cm). The calculator automatically converts to metric before applying the Mifflin-St Jeor formula, which is defined in SI units.
How often should I recalculate my BMR?
Recalculate whenever your weight changes by more than 5 lb (2.3 kg), after a significant change in activity level, or yearly — since age is a continuous variable in the formula. People actively cutting or bulking should recalculate every 4–6 weeks.
Does the formula work for people over 65?
The Mifflin-St Jeor equation can be used for older adults, but accuracy decreases with age due to greater variability in body composition. Studies suggest it may underestimate energy needs in very lean elderly individuals. Consult a dietitian for personalized recommendations in this population.
Sources and references
- Mifflin MD et al. — A new predictive equation for resting energy expenditure in healthy individuals (1990)
- Frankenfield D et al. — Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults (2005)
- NIH — Body Weight Planner
- CDC — Healthy Weight, Nutrition, and Physical Activity