Your Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest just to sustain basic life functions — breathing, circulation, cell repair, and organ function. It is the foundation of every calorie and diet calculation you will ever make.
What Is BMR?
BMR (Basal Metabolic Rate) represents the minimum calories needed to keep your body functioning if you were lying completely still for 24 hours. It accounts for:
- Brain function (~20% of BMR)
- Heart pumping (~7%)
- Liver function (~27%)
- Kidney function (~7%)
- Muscle maintenance at rest (~18%)
- Other organs and processes (~21%)
BMR typically accounts for 60-75% of your total daily calorie expenditure. Even highly active people burn most of their calories through BMR, not exercise.
The Mifflin-St Jeor Formula (Most Accurate)
The Mifflin-St Jeor equation, developed in 1990, is considered the most accurate BMR formula for most adults:
Example (woman, 35 years old, 65 kg, 165 cm):
Example (man, 30 years old, 80 kg, 178 cm):
→ Use our BMR Calculator to get your exact result instantly.
The Harris-Benedict Equation
The original Harris-Benedict formula (1919, revised 1984) is also widely used:
Research shows the Mifflin-St Jeor equation outperforms the original Harris-Benedict by about 5% accuracy. The revised Harris-Benedict (Roza & Shizgal, 1984) performs similarly to Mifflin-St Jeor. Either formula is appropriate for most practical purposes.
From BMR to TDEE: The Activity Factor
BMR alone is not your actual calorie need. To get TDEE (Total Daily Energy Expenditure), multiply by an activity factor:
| Lifestyle | Multiplier | Example (BMR=1,600) |
|---|---|---|
| Sedentary (desk job, no exercise) | × 1.2 | 1,920 kcal |
| Lightly active (exercise 1-3×/wk) | × 1.375 | 2,200 kcal |
| Moderately active (3-5×/wk) | × 1.55 | 2,480 kcal |
| Very active (6-7×/wk) | × 1.725 | 2,760 kcal |
| Extra active (physical job + exercise) | × 1.9 | 3,040 kcal |
TDEE is your maintenance level — the calories needed to keep your current weight stable.
What Affects Your BMR?
BMR is not fixed — it changes with life circumstances:
- Muscle mass — the biggest controllable factor. Muscle burns ~6 kcal/lb/day at rest vs ~2 kcal/lb/day for fat. More muscle = higher BMR
- Age — BMR declines roughly 2-3% per decade after 30, primarily due to muscle loss
- Sex — men have higher BMR than women of the same size, mainly due to greater muscle mass
- Thyroid function — hypothyroidism can reduce BMR by 15-30%; hyperthyroidism can raise it
- Caloric restriction — prolonged dieting reduces BMR by 5-15% through metabolic adaptation
- Body temperature/environment — cold environments slightly increase BMR as the body generates more heat
Using BMR for Weight Loss and Gain Goals
Once you know your TDEE (BMR × activity factor), setting calorie targets is straightforward:
- Weight maintenance: Eat at TDEE
- Lose 1 lb/week: Eat at TDEE − 500 kcal
- Gain 1 lb/week: Eat at TDEE + 500 kcal
Important: BMR and TDEE formulas are estimates with a ±10-15% margin of error. Track your weight for 2-3 weeks at your calculated maintenance calories, then adjust up or down based on what you observe. The most accurate TDEE is the one calibrated to your actual results.
→ Get a full calorie plan including macros with our Calorie Calculator.