What is Basal Metabolic Rate (BMR)?
Basal Metabolic Rate (BMR) is the number of calories your body burns in a 24-hour period when you are at complete rest — not moving, not digesting food, just keeping your organs running. It represents the energy your body needs for the absolute basics of life: breathing, pumping blood, maintaining body temperature, and keeping cells alive.
BMR is important because it forms the foundation of any calorie calculation. Whether your goal is weight loss, muscle gain, or simply maintaining your current weight, you must first know your BMR to calculate how much you should eat.
The Mifflin-St Jeor Formula
This calculator uses the Mifflin-St Jeor equation, developed in 1990 and validated by multiple large-scale studies. The Academy of Nutrition and Dietetics considers it the most accurate formula for the general population.
For Men:
BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5
For Women:
BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161
Example calculation for a 30-year-old man, 75 kg, 178 cm:
- (10 × 75) = 750
- (6.25 × 178) = 1,112.5
- (5 × 30) = 150
- BMR = 750 + 1,112.5 − 150 + 5 = 1,717.5 calories/day
BMR vs. TDEE: What Is the Difference?
BMR is only part of the picture. In real life, you are not lying in a coma — you move, you digest food, you exercise. Your Total Daily Energy Expenditure (TDEE) accounts for all of this.
| Activity Level | Multiplier | Example |
|---|---|---|
| Sedentary | BMR × 1.2 | Desk job, no exercise |
| Lightly active | BMR × 1.375 | Walking, light gym 1–3×/week |
| Moderately active | BMR × 1.55 | Regular exercise 3–5×/week |
| Very active | BMR × 1.725 | Hard training 6–7×/week |
| Extra active | BMR × 1.9 | Athlete or physical labour job |
What Affects Your BMR?
Body composition: Lean muscle mass is metabolically active tissue. A person with 80 kg of lean muscle burns significantly more calories at rest than a person with the same weight but 30% body fat. Each kilogram of muscle burns roughly 13 calories per day at rest; fat burns only 4.5 calories.
Age: BMR declines approximately 2% per decade after age 20. Between ages 30 and 60, this amounts to a reduction of roughly 300-400 calories per day — which explains why many people gain weight eating the same amount they ate in their 20s.
Sex: Men typically have higher BMR than women because they carry more lean muscle mass and have higher levels of testosterone. The Mifflin-St Jeor equation accounts for this with the +5 (male) and −161 (female) constant.
Thyroid function: The thyroid gland produces hormones that directly regulate metabolic rate. Hypothyroidism (underactive thyroid) can lower BMR by 15-40%; hyperthyroidism can raise it by 25-80%. If you suspect thyroid issues, see a doctor — no calculator can detect this.
Genetics: Twin studies suggest that genetic factors account for 40-70% of variation in BMR. Some people genuinely burn more or fewer calories at rest than their measurements predict.
How to Use Your BMR to Set Calorie Goals
For weight loss: Calculate your TDEE (BMR × activity multiplier). Create a deficit of 300–500 calories below TDEE. Never eat below your BMR — doing so causes muscle loss and metabolic slowdown. Aim to lose 0.5–1 kg (1–2 lb) per week.
For muscle gain: Calculate your TDEE. Eat 200–400 calories above TDEE to fuel muscle growth without excessive fat gain. Combine with a progressive resistance training program. Protein intake should be at least 1.6 g per kg of body weight per day.
For weight maintenance: Eat at your TDEE. Monitor weight weekly and adjust calories if weight trends up or down over 3–4 weeks.
Frequently Asked Questions
Is BMR the same as resting metabolic rate (RMR)?
Nearly, but not exactly. BMR is measured under very strict conditions: after 8+ hours of sleep, 12+ hours of fasting, in a temperature-controlled room, with zero physical activity. RMR is measured under more relaxed conditions (just fasted and rested). RMR is typically 10-20% higher than BMR, so most calculators actually estimate RMR. The terms are often used interchangeably in everyday use.
Why is my calculated BMR different from what I was told by a fitness device?
Fitness trackers use different formulas and combine activity data from their sensors to estimate TDEE, not pure BMR. Some use accelerometers, heart rate, and proprietary algorithms. The Mifflin-St Jeor equation is the gold standard for population-level accuracy, but individual variation can be ±10-15%.
Can I increase my BMR permanently?
Yes, by building muscle. Each kilogram of added lean muscle raises BMR by approximately 13 calories per day. While that sounds small, adding 5 kg of muscle (a realistic long-term goal) raises BMR by about 65 calories per day — or about 2,000 extra calories per month your body burns without additional exercise.
Does eating small frequent meals increase BMR?
No. This is a myth. Research consistently shows that meal frequency has no meaningful effect on metabolic rate or weight loss when total calories are equal. What matters for BMR is your total calorie intake and macronutrient composition, not how often you eat. Protein does have a higher thermic effect (20-30%) compared to carbs (5-10%) or fat (0-3%).
How accurate is the Mifflin-St Jeor equation?
For most healthy adults, it predicts BMR within 10% of measured values. It tends to overestimate BMR in obese individuals and underestimate it in very muscular people. Direct measurement (indirect calorimetry) is more accurate but requires specialized equipment only available in clinical settings.