What is a BMR calculator?
BMR stands for Basal Metabolic Rate — the calories your body burns at complete rest, just keeping you alive. Heart beating, lungs breathing, brain thinking, organs maintaining temperature: all of that needs energy, even if you spend the entire day in bed.
BMR is the baseline floor for daily calorie needs. To get your total daily energy expenditure (TDEE) — the actual calories you burn per day — multiply BMR by an activity factor. See the Calorie Calculator for TDEE.
How is BMR calculated?
Several formulas exist. The most accurate for the general population is the Mifflin-St Jeor equation (1990):
Men: BMR = (10 × weight kg) + (6.25 × height cm) − (5 × age) + 5
Women: BMR = (10 × weight kg) + (6.25 × height cm) − (5 × age) − 161
Older formulas like Harris-Benedict (1919) tend to overestimate by 5%. Katch-McArdle uses lean body mass and is more accurate for athletic individuals — but requires body fat % measurement.
This calculator uses Mifflin-St Jeor by default and offers Harris-Benedict and Katch-McArdle as alternates.
Worked example
A 30-year-old man, 175 cm tall, 75 kg:
BMR = (10 × 75) + (6.25 × 175) − (5 × 30) + 5
= 750 + 1,093.75 − 150 + 5
= 1,698.75 kcal/day
A 30-year-old woman, 162 cm tall, 60 kg:
BMR = (10 × 60) + (6.25 × 162) − (5 × 30) − 161
= 600 + 1,012.5 − 150 − 161
= 1,301.5 kcal/day
Even at complete rest, the man burns ~1,700 kcal/day; the woman ~1,300. Most diet plans should never go below BMR for sustained periods.
Why men burn more than women
The formula difference (−161 vs +5) reflects body composition:
- Men typically have higher muscle mass (more metabolic activity)
- Women have higher essential fat (less metabolic activity)
- Hormonal differences (testosterone vs estrogen)
This is averaged into the formula. Individuals vary by ±10% from the predicted value.
Components and inputs
Sex (biological)
Male or female. The formula offset differs.
Age
In years. BMR decreases by ~5 kcal/day for each year of age, as lean mass tends to decline.
Height
In cm or feet/inches. Affects body surface area.
Weight
In kg or lb. Affects metabolic mass.
Optional — body fat %
Switches to Katch-McArdle:
BMR = 370 + (21.6 × lean body mass kg)
where lean body mass = weight × (1 − body fat fraction)
This is more accurate for athletes (high muscle, low fat) and obese individuals (muscle metabolizes; fat barely does).
Variants of the formula
| Formula | Year | Best for |
|---|---|---|
| Mifflin-St Jeor | 1990 | General population (default) |
| Harris-Benedict (revised) | 1984 | Backward compatibility |
| Katch-McArdle | 1975 | Athletes / lean body mass known |
| Cunningham | 1980 | Athletes (similar to Katch-McArdle) |
| Schofield | 1985 | WHO / public health (uses height, weight, age) |
For most people, Mifflin-St Jeor is accurate within ±10%.
What BMR is NOT
- It's NOT your daily calorie need (multiply by activity factor for that)
- It's NOT a weight-loss prescription
- It's NOT what you burn during exercise (that's exercise expenditure, separate)
- It's NOT a substitute for measured RMR (indirect calorimetry in a clinical setting is more accurate)
What affects BMR
| Factor | Effect | Notes |
|---|---|---|
| Lean muscle mass | Increases | Muscle burns 13 kcal/kg/day at rest |
| Fat mass | Slight increase | Fat burns 4.5 kcal/kg/day at rest |
| Age | Decreases | ~5 kcal/year decline after age 30 |
| Sex (male) | Higher | Due to muscle mass |
| Body temperature | Increases | Fever raises BMR ~7% per °C |
| Thyroid | Major | Hyperthyroid raises BMR; hypo lowers it |
| Pregnancy | Increases | Up to 20% in third trimester |
| Caffeine / nicotine | Slight increase | Temporary |
| Crash dieting | Decreases | Body conserves energy when starved |
| Sleep | Slightly lower | BMR is measured awake but inactive |
Worked example: weight loss math
A 70 kg woman, 30 y, 165 cm has BMR ≈ 1,400 kcal. Her TDEE (light activity) is 1,400 × 1.375 = 1,925 kcal.
To lose 0.5 kg/week (~3,500 kcal deficit):
Daily deficit = 3,500 / 7 = 500 kcal
Target intake = 1,925 − 500 = 1,425 kcal/day
That's at BMR — sustainable for weeks, not months. For longer-term sustainable loss, consider a smaller deficit (300 kcal/day → ~1.5 kg/month).
Never drop intake significantly below BMR. The body downregulates metabolism and you regain rapidly when you stop.
Considerations
- BMR is a prediction, not a measurement. Real RMR (resting metabolic rate, measured via indirect calorimetry) is the gold standard but requires lab equipment.
- ±10% accuracy. Two people with identical inputs can have 200 kcal different actual BMR.
- Day-to-day variation. Even your own BMR varies by 5-10% based on sleep, stress, hormones, hydration.
- Don't measure body composition with bathroom scales. Bioimpedance scales are noisy; DEXA / hydrostatic weighing / skinfold calipers are more accurate.
Limitations
- Doesn't account for medical conditions (thyroid disease, diabetes, PCOS) that shift BMR by 10-30%.
- Doesn't apply to children — separate growth-adjusted equations exist.
- Doesn't predict the adaptive thermogenesis drop after sustained dieting (BMR can drop 15-25% with severe caloric restriction).
- Not a substitute for medical advice — see a doctor or registered dietitian for individualized planning.
Related calculators
- Calorie / TDEE — BMR × activity factor
- BMI — weight categorization
- Body Fat — % composition
- Ideal Weight — target weight range
- Macro Split — protein/carbs/fat breakdown
- Water Intake — daily hydration
Final note. BMR is the calorie floor — what your body needs to stay alive at rest. Multiply by activity factor to get TDEE; eat above TDEE to gain, below to lose. Don't dip too far below BMR for sustained periods — your metabolism rebels. For most people, Mifflin-St Jeor + activity multiplier gets within ±10% of reality, which is enough for practical diet planning.