BMR Calculator
For informational purposes only. Consult a healthcare professional before making dietary changes.
For informational purposes only. Consult a healthcare professional before making significant dietary changes.
What is Basal Metabolic Rate?
Your Basal Metabolic Rate (BMR) is the number of calories your body burns every 24 hours while at complete rest — no movement, no digestion, no activity of any kind. It represents the energy cost of keeping you alive: your heart beating, your lungs breathing, your liver filtering, your kidneys processing, and your cells continuously repairing themselves.
For most people, BMR accounts for 60–75% of total daily energy expenditure. Even if you are completely sedentary, your body is burning a substantial number of calories just to maintain its basic operations. This is why extreme low-calorie diets are problematic — they can push intake below BMR, which signals the body to slow down these baseline processes and break down muscle for energy.
BMR Formulas Compared
Two equations are widely used to estimate BMR. Both use weight, height, age, and sex as inputs.
Mifflin-St Jeor (1990) — Recommended
Men: BMR = (10 × kg) + (6.25 × cm) − (5 × age) + 5
Women: BMR = (10 × kg) + (6.25 × cm) − (5 × age) − 161
Harris-Benedict (Revised 1984)
Men: BMR = 88.362 + (13.397 × kg) + (4.799 × cm) − (5.677 × age)
Women: BMR = 447.593 + (9.247 × kg) + (3.098 × cm) − (4.330 × age)
| Formula | Year | Accuracy (within 10%) | Notes |
|---|---|---|---|
| Mifflin-St Jeor | 1990 | 82% of subjects | Preferred for non-obese adults; based on a more modern study population |
| Harris-Benedict (rev.) | 1984 | 81% of subjects | Slightly higher estimates than Mifflin; still widely used |
| Harris-Benedict (orig.) | 1919 | ~70% of subjects | Tends to overestimate by 5-15%; based on early 20th century subjects |
Worked Example — Mifflin-St Jeor
Calculate BMR for a 28-year-old man, 80 kg, 178 cm:
What Affects BMR?
The four inputs in the formula explain most BMR variation, but several other factors influence your actual metabolic rate:
Body composition. Muscle tissue is metabolically active — it burns calories even at rest. Fat tissue is relatively inert. Two people with the same weight but different muscle-to-fat ratios will have measurably different BMRs. A person with 70 kg and 15% body fat will burn more calories at rest than someone with 70 kg and 30% body fat.
Thyroid function. The thyroid gland regulates metabolic rate through hormones (T3, T4). Hypothyroidism (underactive thyroid) can reduce BMR by 10–40%, while hyperthyroidism increases it. If you gain weight consistently despite eating at your calculated maintenance level, thyroid testing is worth discussing with a doctor.
Genetics. Studies on identical twins show that roughly 40–70% of BMR variation not explained by body composition has a genetic component. Some individuals naturally burn 200–400 more or fewer calories per day than the formula predicts.
Temperature. Living in cold environments increases BMR slightly as the body works to maintain core temperature. This effect is modest for most people in modern heated homes.
BMR vs TDEE vs Calorie Goal
BMR is a theoretical lower bound — the minimum calories needed if you were in a medically induced coma. You should never eat below your BMR without medical supervision. For practical dietary planning, use TDEE (BMR × activity factor) as your maintenance calories, then adjust up or down based on your goal.
| Metric | What It Means | Use For |
|---|---|---|
| BMR | Calories at complete rest | Understanding metabolic baseline; never eat below this |
| TDEE | BMR × activity factor | Maintenance calories for your actual lifestyle |
| Deficit goal | TDEE - 300 to 500 | Gradual fat loss (~0.3-0.5 kg/week) |
| Surplus goal | TDEE + 200 to 300 | Lean muscle building with minimal fat gain |
Frequently Asked Questions
What is Basal Metabolic Rate (BMR)?
BMR is the number of calories your body needs to sustain basic physiological functions while at complete rest — breathing, blood circulation, body temperature regulation, cell production, and organ function. It represents the minimum energy required to keep you alive if you stayed in bed all day. BMR accounts for 60-75% of total daily energy expenditure for most sedentary adults.
Which BMR formula is more accurate — Mifflin-St Jeor or Harris-Benedict?
The Mifflin-St Jeor equation (1990) is generally considered more accurate for modern populations. A 2005 study published in the Journal of the American Dietetic Association found that Mifflin-St Jeor predicted BMR within 10% of measured values for 82% of the test subjects, compared to 81% for the revised Harris-Benedict (1984). Both are population-level estimates; individual variation can still be 200-400 calories. The original Harris-Benedict (1918) tends to overestimate by 5-15% and is rarely used today.
How do I use BMR to find my daily calorie needs?
Multiply your BMR by an activity factor (also called PAL, Physical Activity Level). Sedentary adults multiply by 1.2, lightly active by 1.375, moderately active by 1.55, very active by 1.725, and extra active by 1.9. The result is your TDEE — Total Daily Energy Expenditure — which represents total calories burned including exercise and daily movement. To lose weight, eat below TDEE; to gain weight, eat above it.
Can I increase my BMR?
Yes. The most effective ways to increase BMR are: (1) Building muscle through resistance training — muscle tissue burns 6 calories per pound per day at rest vs. 2 calories for fat. (2) Eating adequate protein — the thermic effect of protein is 20-30% (your body burns 20-30% of its calories in digestion), compared to 5-10% for carbs and 0-3% for fat. (3) Staying hydrated — even mild dehydration reduces metabolic rate. (4) Getting enough sleep — sleep deprivation reduces BMR and increases appetite hormones.
Why is my BMR lower than expected?
Several factors can reduce BMR below the formula estimate: prolonged caloric restriction (the body adapts by burning fewer calories — often called "metabolic adaptation"), low muscle mass (the elderly, sedentary individuals), thyroid disorders (hypothyroidism reduces metabolic rate by 10-40%), hormonal imbalances, or naturally low metabolic rate genetic variation. If you consistently gain weight despite eating at your calculated TDEE, consider consulting an endocrinologist to rule out thyroid or other hormonal issues.
Related Calculators
- Calorie Calculator — Daily calorie needs with activity level (TDEE)
- TDEE Calculator — Total Daily Energy Expenditure in detail
- BMI Calculator — Body Mass Index with WHO categories
- Macro Calculator — Protein, carb, and fat targets from your TDEE