BMR Calculator | Free Basal Metabolic Rate Tool with 3 Formulas
Calculate your Basal Metabolic Rate using 3 medical formulas side by side. Understand how age, gender, muscle mass, and genetics affect your metabolism. Then use our TDEE Calculator to plan weight changes.
📖 Want to understand your results?
Read our complete guide to the BMR Calculator — with real-world examples, expert insights, and pro tips.
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What is BMR (Basal Metabolic Rate)?
Your Basal Metabolic Rate (BMR) is the minimum number of calories your body needs to function at complete rest. It powers your breathing, circulation, cell production, and temperature regulation. BMR accounts for 60-75% of your total daily calorie burn.
Unlike TDEE (which includes digestion and physical activity), BMR is what you'd burn if you stayed in bed all day. It's the baseline upon which all other calorie calculations are built.
The 3 BMR Formulas Compared
⭐ Mifflin-St Jeor (1990)
The most accurate formula for the general population. Developed using modern metabolic data and recommended by the Academy of Nutrition and Dietetics.
Men: 10W + 6.25H - 5A + 5
Women: 10W + 6.25H - 5A - 161
📜 Harris-Benedict (1919/1984)
The original standard formula. Revised in 1984 for modern populations. Slightly overestimates in overweight individuals but well-validated.
Men: 13.397W + 4.799H - 5.677A + 88.362
Women: 9.247W + 3.098H - 4.330A + 447.593
💪 Katch-McArdle (1975)
The most precise formula — if you know your body fat %. Uses lean body mass instead of total weight. Preferred by athletes and bodybuilders.
BMR = 370 + (21.6 × LBM)
LBM = Lean Body Mass (kg)
How BMR Changes with Age
BMR naturally declines by about 1-2% per decade after age 20. This is primarily due to gradual muscle loss (sarcopenia) and hormonal changes. However, this decline is not inevitable — strength training and adequate protein intake can largely prevent it.
| Age Range | Typical BMR Change | Primary Cause |
|---|---|---|
| 20-30 | Peak, stable | Highest muscle mass |
| 30-40 | -1 to -2% | Gradual muscle loss begins |
| 40-50 | -3 to -5% | Hormonal changes (especially women) |
| 50-60 | -5 to -10% | Menopause, reduced activity |
| 60+ | -10 to -15% | Muscle loss accelerates if untrained |
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BMR vs TDEE: What's the Difference?
🔥 BMR (This Calculator)
- Calories at complete rest
- 60-75% of daily burn
- Used to understand your metabolism
- Primary factor: lean body mass
- Best for: learning how your body works, comparing formulas
📊 TDEE (Separate Tool)
- BMR + digestion (TEF) + activity
- Your total daily burn
- Used to plan weight changes
- Primary factor: activity level
- Best for: setting calorie targets, macros, tracking adaptation
BMR Calculator Pros & Cons
Pros
- ✅ 3 formulas side by side (Mifflin, Harris, Katch)
- ✅ BMR-by-age chart included
- ✅ Free forever — no subscriptions
- ✅ Works offline after page load
- ✅ No sign-up or email required
Cons
- ✗ No Excel/CSV export option
- ✗ No dedicated mobile app
Reviewed by Dr. Sarah Mitchell
Health & Nutrition SpecialistRegistered dietitian and exercise physiologist with 12+ years of clinical experience.
Frequently Asked Questions
What is BMR and how is it different from TDEE?
BMR (Basal Metabolic Rate) is the calories your body needs at complete rest — just breathing, circulation, and cell function. It accounts for 60-75% of your daily burn. TDEE (Total Daily Energy Expenditure) adds digestion (TEF ~10%) and all physical activity on top of BMR.
Which BMR formula is most accurate?
The Mifflin-St Jeor equation is most accurate for the general population with about 90% accuracy within 10% of measured BMR. Katch-McArdle is more accurate if you know your body fat %. Harris-Benedict is the classic standard but slightly overestimates for overweight individuals.
Does age affect BMR?
Yes. BMR decreases by about 1-2% per decade after age 20. This is primarily due to loss of muscle mass. Strength training can largely prevent this decline by preserving muscle tissue.
What is the difference between BMR and RMR?
BMR is measured under strict conditions (complete rest, 12-hour fast, comfortable temperature). RMR (Resting Metabolic Rate) is measured under less strict conditions and is typically 10-20% higher than BMR. For practical purposes, they're used interchangeably in most calculators.
Can exercise increase my BMR?
Yes, indirectly. While the calories burned during exercise contribute to TDEE, building muscle through strength training increases your BMR long-term. Muscle burns about 13 kcal/kg/day at rest vs fat at about 4.5 kcal/kg/day.
What is metabolic adaptation?
Metabolic adaptation is when your BMR decreases in response to prolonged calorie restriction. It can reduce BMR by 15-30%. This is why gradual, sustainable deficits with periodic diet breaks are more effective than crash diets.
How accurate are BMR calculators?
Formula-based BMR estimates are typically within 10% of measured BMR via indirect calorimetry. Your actual BMR can vary due to genetics, hormones, muscle mass, and metabolic adaptations. Use the estimate as a starting point and adjust based on real-world results.
How do I find my body fat % for Katch-McArdle?
The most accurate methods are DEXA scans, hydrostatic weighing, and caliper measurements. For a rough estimate, you can use our Body Fat Calculator which uses the US Navy circumference method.
What is a normal BMR by age?
A typical BMR for a 30-year-old man at 70kg/175cm is ~1,650 kcal/day. For a 30-year-old woman at 60kg/165cm it's ~1,320 kcal/day. These numbers decrease about 1-2% per decade. Our calculator shows a detailed BMR-by-age chart for your specific stats.
How do I use my BMR to lose weight?
To lose weight, you need to eat below your TDEE (not your BMR). A 300-500 kcal/day deficit below TDEE results in sustainable 0.25-0.5 kg loss per week. Use our TDEE Calculator to set precise calorie and macro targets.
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