BMR Calculator
Your basal metabolic rate is what you would burn in a day if you stayed in bed without moving, the energy cost of keeping the heart, lungs, brain and organs running. Enter age, sex, height and weight and the calculator uses the Mifflin-St Jeor equation (more accurate than the old Harris-Benedict for modern populations) to estimate daily BMR in calories. Multiply by an activity factor and you get TDEE, the figure diets actually care about.
How BMR is estimated
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1
Enter age, sex, height and weight
Metric or imperial. Biological sex is used because male and female bodies differ in lean mass proportion.
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2
Mifflin-St Jeor equation runs
For men: BMR = 10·kg + 6.25·cm − 5·age + 5. For women: BMR = 10·kg + 6.25·cm − 5·age − 161.
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3
Adjust with activity factor (optional)
Choose sedentary (×1.2) through very active (×1.9) to estimate total daily energy expenditure.
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4
Read BMR and TDEE
BMR is your at-rest burn. TDEE adds movement. Diets target TDEE minus a deficit for weight loss, TDEE plus a surplus for gain.
Mifflin-St Jeor (1990): the modern standard
- Men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5
- Women: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) − 161
This equation replaced Harris-Benedict (1919) as the clinical default because it fits contemporary populations within ±10% across a wider range of body types.
Activity factors (Katch-McArdle style)
| Lifestyle | Multiplier |
|---|---|
| Sedentary (desk job, no exercise) | 1.20 |
| Lightly active (1-3 workouts/week) | 1.375 |
| Moderately active (3-5 workouts/week) | 1.55 |
| Very active (6-7 workouts/week) | 1.725 |
| Extremely active (physical job + daily training) | 1.90 |
Example
A 35-year-old woman, 165 cm, 68 kg, moderately active:
- BMR = (10 × 68) + (6.25 × 165) − (5 × 35) − 161
- BMR = 680 + 1031.25 − 175 − 161 ≈ 1,375 kcal/day
- TDEE at 1.55 = 2,131 kcal/day
To lose about 0.5 kg per week, target 2,131 − 500 = 1,631 kcal/day.
Limitations
- Body composition matters: Mifflin assumes average lean mass. Very lean or very muscular people can be off by 200+ kcal.
- Adaptation: after weeks of deficit, metabolism slows (adaptive thermogenesis). Recalculate every few kilos lost.
- Medical conditions (thyroid, medication, illness) shift BMR independently of the equation.
Frequently Asked Questions
Mifflin-St Jeor is the current default in clinical settings because it fits modern body types more closely. Harris-Benedict tends to overestimate by 5-10% on average.
Fitness trackers combine a BMR estimate with step counts and heart rate, and many use Katch-McArdle (which needs body fat percentage). Differences of ±15% are normal across methods.
It drops roughly 1-2% per decade after 20, mostly due to loss of lean mass. Resistance training slows this decline significantly.
Close but not identical. BMR is measured after a full night’s sleep, 12 hours fasted, in a neutral environment. RMR is measured under looser conditions and usually lands about 10% higher.
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