How the BMR calculator estimates your resting calorie burn
Your Basal Metabolic Rate is the number of calories your body burns at complete rest just to keep you alive: breathing, pumping blood, maintaining body temperature, repairing cells. It typically accounts for 60–75% of your total daily calorie expenditure, making it the foundation for any nutrition or weight management plan.
This calculator shows your BMR from three well-established formulas simultaneously so you can compare them. The primary result uses Mifflin-St Jeor (the most accurate for most adults), with Harris-Benedict and Katch-McArdle shown alongside.
How to use it
- Enter your age and sex.
- Enter your weight and height (the unit selector is grouped with each field).
- Your BMR appears instantly from all three formulas, plus your estimated sedentary TDEE (BMR × 1.2).
How BMR is calculated
Mifflin-St Jeor (primary, recommended)
Developed in 1990, this is the formula the American Dietetic Association recommends for healthy adults. It predicts measured resting metabolic rate within 10% for most people.
Harris-Benedict (revised)
The original 1919 formula, revised by Roza and Shizgal in 1984. It tends to run slightly higher than Mifflin-St Jeor, particularly for overweight individuals.
Katch-McArdle
This formula uses lean body mass instead of total weight, making it more accurate for very lean or very muscular people. It requires knowing your body fat percentage (from a DEXA scan or body fat calculator). The formula is the same for both sexes.
Since this calculator doesn't ask for body fat directly, it estimates lean mass from your BMI using the Deurenberg approximation. For a more precise Katch-McArdle result, measure your body fat with our body fat calculator first.
Why results differ between formulas
The three formulas were derived from different study populations and use different variables. Mifflin-St Jeor and Harris-Benedict use weight, height, age, and sex. Katch-McArdle uses only lean mass, ignoring the other variables. For most people the results cluster within 50–100 kcal of each other. If they diverge significantly for you, consider which formula best matches your body composition:
- Lean and muscular — Katch-McArdle is usually most accurate (lean mass drives metabolism).
- Average body composition — Mifflin-St Jeor is the best general predictor.
- Older adults — Harris-Benedict may slightly overestimate; prefer Mifflin-St Jeor.
From BMR to daily calories (TDEE)
BMR alone isn't your calorie budget. You burn additional energy through daily movement, exercise, and digesting food. Multiply BMR by an activity factor to estimate Total Daily Energy Expenditure:
| Activity level | Multiplier | Example |
|---|---|---|
| Sedentary | 1.2 | Desk job, no exercise |
| Lightly active | 1.375 | Walking, 1–3 light workouts/week |
| Moderately active | 1.55 | 3–5 sessions/week |
| Active | 1.725 | Daily hard exercise |
| Very active | 1.9 | Physical job + daily training |
The "Sedentary TDEE" shown in the results is BMR × 1.2. For a full TDEE calculation with activity level selection, use our calorie calculator.
What affects your BMR
- Muscle mass — the single biggest modifiable factor. Each kg of muscle burns about 13 kcal/day at rest versus 4.5 kcal for fat.
- Age — BMR declines roughly 1–2% per decade after 20, mostly due to muscle loss.
- Sex — men have higher BMR at equal weight/height due to more muscle and less essential fat.
- Genetics and thyroid function — can shift true BMR by 200–300 kcal from predicted values.
- Caloric restriction — prolonged dieting lowers BMR through metabolic adaptation (adaptive thermogenesis). This is temporary and reverses when intake normalizes.
Common mistakes to avoid
- Eating below your BMR. Your BMR is the floor your body needs for basic function. Deficits should be taken from TDEE, not below BMR, to avoid muscle loss and hormonal disruption.
- Using BMR as your calorie target. BMR is not how many calories you should eat. You move during the day, so your actual needs (TDEE) are always higher.
- Assuming BMR is fixed. It changes as your weight, muscle mass, and age change. Recalculate after significant body composition changes.
- Conflating BMR with RMR. Resting Metabolic Rate (RMR) is measured after rest but not full fasting conditions, so it runs 5–10% higher than true BMR. Most "metabolic tests" at gyms measure RMR, not BMR.
Limitations
All BMR formulas are population-level predictions. Individual variation from genetics, hormones, medication, and body composition means your true BMR could differ by 100–300 kcal. For clinical precision, indirect calorimetry (a breathing test at a metabolic lab) measures your actual resting energy expenditure. These calculator results are best used as a planning starting point, adjusted based on real-world results over 2–3 weeks.
Frequently asked questions
What is the difference between BMR and TDEE?
BMR (Basal Metabolic Rate) is the energy your body uses at complete rest — lying still, awake, fasting. TDEE (Total Daily Energy Expenditure) is BMR multiplied by your activity level, and represents total calories burned in a day including movement, exercise, and digestion. For weight management, TDEE is the more practical number.
Which BMR formula should I use?
Mifflin-St Jeor is the most accurate for most adults — studies show it predicts within 10% of measured values. Use Harris-Benedict if you want a historical comparison. Use Katch-McArdle only if you know your body fat percentage (from a DEXA scan or reliable method), as it accounts for lean mass and is most accurate for very lean or very muscular individuals.
Why does BMR decrease with age?
Primarily because of muscle loss (sarcopenia). Muscle tissue is metabolically active and burns calories at rest; as you lose it, BMR drops. Hormonal changes also contribute. The decline is roughly 1–2% per decade after 20, but resistance training significantly slows muscle loss and preserves a higher BMR.
Can I increase my BMR?
Yes, through building muscle mass. Each kilogram of muscle burns roughly 13 kcal/day at rest, compared to about 4.5 kcal/day for fat. Resistance training is the most effective way to increase BMR long term. Short-term boosts from caffeine, cold exposure, or high-protein meals exist but are small and temporary.
Why is female BMR lower than male BMR?
Women typically have less muscle mass and more essential body fat than men at the same weight and height, and muscle is more metabolically active. The Mifflin-St Jeor formula accounts for this with a 166 kcal/day offset between sexes. Hormonal differences (estrogen, testosterone) also play a role in body composition.
Should I eat below my BMR to lose weight?
Generally no. Eating below BMR can trigger excessive muscle loss, hormonal disruption (especially in women), fatigue, and metabolic adaptation that slows further loss. A deficit of 300–500 kcal below your TDEE (not BMR) is safer and more sustainable. Your TDEE is always higher than your BMR because you move during the day.
How accurate are BMR calculators?
Prediction equations are typically within 10% of measured values (indirect calorimetry) for healthy adults. Individual variation due to genetics, thyroid function, and body composition can push results further off. For precise numbers, ask about a metabolic test at a sports lab or hospital.