Free, accurate BMR calculator using Mifflin-St Jeor and Harris-Benedict formulas. Calculate your metabolism and daily calorie needs. Updated for 2025.
📚 Complete Guide to BMR and Metabolism
What is Basal Metabolic Rate (BMR)?
BMR is the number of calories your body burns at complete rest to maintain essential functions:
- Breathing and circulation: ~20% of BMR
- Brain and nervous system: ~20% of BMR
- Kidney and liver function: ~20% of BMR
- Cell production and protein synthesis: ~20% of BMR
- Temperature regulation: ~10% of BMR
- Other metabolic processes: ~10% of BMR
BMR represents 60-75% of your total daily calorie expenditure. Even if you did absolutely nothing all day, your body would still burn these calories just to stay alive.
📊 Example: A 30-year-old woman (165cm, 70kg) has a BMR of ~1,450 calories. This means her body burns 1,450 calories per day just existing, before any physical activity or exercise.
BMR Formula Comparison
1. Mifflin-St Jeor Equation (1990) - MOST ACCURATE
Developed in 1990, this is the most accurate formula for modern populations with ±10% error margin.
- 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
2. Harris-Benedict Equation (Revised 1984)
Original equation from 1919, revised in 1984. Less accurate than Mifflin-St Jeor for modern populations.
- Men: BMR = 88.362 + (13.397 × weight kg) + (4.799 × height cm) - (5.677 × age)
- Women: BMR = 447.593 + (9.247 × weight kg) + (3.098 × height cm) - (4.330 × age)
3. Katch-McArdle Formula (Body Fat Based)
Most accurate IF you know your body fat percentage. Doesn't differentiate by gender.
- BMR = 370 + (21.6 × lean body mass in kg)
- Lean body mass = weight × (1 - body fat %)
- Example: 70kg person with 20% body fat = 70 × 0.8 = 56kg lean mass
- BMR = 370 + (21.6 × 56) = 1,580 calories
⚠️ Important: All formulas are estimates with ±10-20% error margin. Individual variations include: genetics, thyroid function, previous dieting history, medication effects, and metabolic adaptations. Use calculator results as a starting point, then adjust based on real-world results.
Understanding TDEE (Total Daily Energy Expenditure)
TDEE is your total daily calorie burn including all activity. It consists of:
TDEE Components:
- BMR (60-75%): Calories for basic body functions
- NEAT (15-30%): Non-Exercise Activity Thermogenesis (walking, fidgeting, standing)
- EAT (5-10%): Exercise Activity Thermogenesis (planned workouts)
- TEF (10%): Thermic Effect of Food (calories burned digesting food)
Activity Multipliers:
| Activity Level |
Multiplier |
Description |
| Sedentary |
1.2 |
Desk job, little/no exercise |
| Lightly Active |
1.375 |
Light exercise 1-3 days/week |
| Moderately Active |
1.55 |
Moderate exercise 3-5 days/week |
| Very Active |
1.725 |
Hard exercise 6-7 days/week |
| Extremely Active |
1.9 |
Physical job + daily training |
How BMR Changes with Age, Gender & Body Composition
Age-Related Decline:
- Ages 20-30: Peak metabolic rate
- Ages 30-40: -2% per decade (BMR drops ~50-100 cal/decade)
- Ages 40-50: -2-3% per decade
- Ages 50-60: -3-5% per decade (accelerated due to menopause in women)
- Ages 60+: -5-8% per decade
By age 70, BMR may be 20-30% lower than at age 30 (300-500 fewer calories/day).
Gender Differences:
- Men have 5-10% higher BMR than women (same age, height, weight)
- Difference primarily due to muscle mass and testosterone
- Average 30-year-old: Male BMR ~1,680 cal vs Female BMR ~1,520 cal (160 cal difference)
- Women's BMR increases 5-10% during menstruation (100-150 extra calories)
- Pregnancy increases needs by 300-500 cal/day
- Breastfeeding increases needs by 500 cal/day
Body Composition Impact:
- Muscle burns 6 cal/lb/day at rest
- Fat burns 2 cal/lb/day at rest
- Gaining 5kg muscle = +66 cal/day BMR increase
- Losing 5kg muscle = -66 cal/day BMR decrease
- This is why crash dieting (which loses muscle) damages metabolism
How to Increase Your BMR
1. Build Muscle Mass (Most Effective)
- Strength training 3-4x per week
- Progressive overload (gradually increase weight/reps)
- Compound exercises: squats, deadlifts, bench press, rows
- Potential BMR increase: 7-8% over 6 months (~100-150 cal/day)
2. Eat Adequate Protein
- Protein has 20-30% thermic effect (you burn 20-30% of protein calories digesting it)
- Supports muscle growth and retention
- Recommended: 1.6-2.2g per kg bodyweight (112-154g for 70kg person)
- Distribute protein across meals (30-40g per meal)
3. Stay Hydrated
- Dehydration can slow metabolism by 2-3%
- Drink 2-3 liters water daily
- Cold water may provide small additional thermogenic effect
4. High-Intensity Interval Training (HIIT)
- Creates "afterburn effect" (EPOC - Excess Post-exercise Oxygen Consumption)
- Burns additional 50-150 calories for 24-48 hours post-workout
- Example: 20 min HIIT = 200 cal during + 100 cal afterburn = 300 total
5. Get Adequate Sleep
- Poor sleep (< 7 hours) reduces BMR and increases hunger hormones
- Sleep deprivation can reduce BMR by 5-10%
- Aim for 7-9 hours quality sleep
6. Avoid Crash Dieting
- Very low calorie diets (<1,200 cal) cause metabolic adaptation
- Can reduce BMR by 10-25% (200-400 calories)
- Effect can last months or years
- Safe deficit: 500-750 cal below TDEE for weight loss
💡 Realistic Expectations: You can increase BMR by 5-10% (75-150 calories/day) over 3-6 months with consistent strength training and proper nutrition. This compounds over time - an extra 100 cal/day = 36,500 cal/year = ~5kg fat loss per year.
Using BMR for Weight Goals
For Weight Loss:
- Calculate TDEE (BMR × activity level)
- Create calorie deficit of 500-750 cal/day
- Expected loss: 0.5-1kg per week
- Never eat below BMR for extended periods (causes metabolic damage)
- Minimum safe intake: Women 1,200 cal, Men 1,500 cal
For Muscle Gain:
- Calculate TDEE
- Add 250-500 cal surplus
- Expected gain: 0.25-0.5kg per week (half muscle, half fat)
- Ensure adequate protein (1.6-2.2g per kg)
- Progressive strength training essential
For Weight Maintenance:
- Eat at your TDEE
- Track weight for 2-4 weeks
- Adjust by 100-200 cal if gaining/losing unintentionally
💡 Expert Tips & Common Mistakes
Top 10 BMR & Metabolism Tips:
- Don't eat below BMR: This causes metabolic damage and muscle loss. Always eat at least BMR calories.
- Track for 2-4 weeks: Use BMR as starting point, then adjust based on real weight changes.
- Be honest about activity level: Most people overestimate - leads to weight gain.
- Prioritize protein: High thermic effect and preserves muscle during weight loss.
- Lift heavy things: Muscle is metabolically expensive - burns calories 24/7.
- Don't fear carbs: Your body burns same calories regardless of macro split (for same total calories).
- Meal timing doesn't matter much: Total daily calories matter more than when you eat them.
- Expect plateaus: As you lose weight, BMR decreases. Recalculate every 5kg loss.
- Don't rely on exercise alone: Can't out-train a bad diet. Exercise burns less than you think.
- Be patient: Sustainable fat loss is 0.5-1kg/week. Faster = muscle loss and metabolic damage.
Common Mistakes to Avoid:
❌ Eating well below BMR: Creates huge deficit causing metabolic adaptation, muscle loss, fatigue, and eventual weight regain. Minimum: women 1,200 cal, men 1,500 cal.
❌ Overestimating activity level: Thinking you're "Very Active" when you exercise 3x/week but sit 8 hours daily. Be conservative with activity multiplier.
❌ Expecting calculator to be exact: BMR formulas have ±10-20% error. Use as starting point, adjust based on results after 2-4 weeks.
❌ Ignoring NEAT: Non-exercise activity (walking, standing, fidgeting) burns more calories than formal exercise for most people. Move throughout the day!
❌ Doing only cardio: Cardio burns calories during exercise but doesn't increase BMR. Strength training increases 24/7 calorie burn by building muscle.
❌ Not recalculating after weight loss: Losing 10kg means your BMR is now lower. Recalculate every 5kg change.
❌ Blaming "slow metabolism": Metabolic differences between people are small (±200 cal). Most weight issues are tracking errors or activity overestimation.
❌ Trying to boost metabolism with pills: No supplement significantly increases metabolism. Focus on muscle building, protein, sleep, and activity.
❓ Frequently Asked Questions
What is BMR and why does it matter?
BMR (Basal Metabolic Rate) is the number of calories your body burns at complete rest to maintain vital functions: breathing, circulation, cell production, nutrient processing, and temperature regulation. It accounts for 60-75% of your total daily calorie burn. BMR matters because it's the foundation for calculating your total daily calorie needs (TDEE). Knowing your BMR helps you set appropriate calorie targets for weight loss, maintenance, or muscle gain. BMR is affected by age (decreases ~2% per decade after 30), sex (men typically 5-10% higher), body composition (muscle burns more than fat), genetics, and hormones. Understanding BMR prevents undereating which can slow metabolism and cause muscle loss.
Which BMR formula is most accurate?
Mifflin-St Jeor equation (1990) is considered most accurate for modern populations, with ~10% error margin. It replaced the older Harris-Benedict equation (1919, revised 1984). Formulas: Mifflin-St Jeor: Men = (10 × weight kg) + (6.25 × height cm) - (5 × age) + 5, Women = (10 × weight kg) + (6.25 × height cm) - (5 × age) - 161. Katch-McArdle (uses body fat %) is most accurate if you know your lean body mass: BMR = 370 + (21.6 × lean mass kg). For average individuals, Mifflin-St Jeor works well. Athletes or very muscular people should use Katch-McArdle. All formulas are estimates - actual BMR can vary ±10-20% due to genetics, hormones, and metabolic adaptations.
How does BMR change with age?
BMR naturally declines with age, decreasing approximately 2-3% per decade after age 30, or about 150-200 calories per decade. This happens for several reasons: Loss of muscle mass (sarcopenia) - muscle burns more calories than fat. Hormonal changes - declining testosterone, estrogen, growth hormone, and thyroid function. Reduced organ function - organs like liver and brain burn fewer calories. Less physical activity leading to further muscle loss. By age 70, BMR may be 20-30% lower than at age 30. To combat declining BMR: maintain muscle through strength training (2-3 times weekly), eat adequate protein (1.2-1.6g per kg bodyweight), stay active with regular cardio, ensure sufficient sleep (7-9 hours), manage stress which affects cortisol and metabolism.
Can I increase my BMR?
Yes, you can increase BMR through several strategies: (1) Build muscle - muscle tissue burns 6 calories per lb daily vs 2 calories for fat. Strength training 3-4 times weekly can increase BMR by 7-8% (~100-150 calories daily). (2) Eat enough protein - high thermic effect (20-30% of calories burned during digestion) and supports muscle growth. Aim for 1.6-2.2g per kg bodyweight. (3) Stay hydrated - dehydration can slow metabolism by 2-3%. Drink cold water for small additional thermogenic effect. (4) HIIT workouts - create 'afterburn' effect (EPOC) burning calories for 24-48 hours post-workout. (5) Get adequate sleep - poor sleep reduces BMR and increases hunger hormones. (6) Avoid crash dieting - very low calories cause metabolic adaptation, lowering BMR by 10-25%. Realistic increase: 5-10% over 3-6 months with consistent effort.
How do I calculate my total daily calorie needs?
Calculate TDEE (Total Daily Energy Expenditure) by multiplying BMR by activity factor: Sedentary (desk job, little exercise) = BMR × 1.2, Lightly active (light exercise 1-3 days/week) = BMR × 1.375, Moderately active (moderate exercise 3-5 days/week) = BMR × 1.55, Very active (hard exercise 6-7 days/week) = BMR × 1.725, Extremely active (physical job + training) = BMR × 1.9. Example: 35-year-old woman, 70kg, 165cm has BMR ~1,450 calories. If lightly active: TDEE = 1,450 × 1.375 = 1,994 calories to maintain weight. For weight loss, subtract 500-750 calories (1,244-1,494 daily). For muscle gain, add 250-500 calories (2,244-2,494 daily). Track weight weekly and adjust if not seeing expected 0.5-1kg change per week.
Does BMR differ between men and women?
Yes, men typically have 5-10% higher BMR than women of the same age, height, and weight. This is primarily due to body composition differences: men naturally have higher muscle mass (muscle burns more calories) and lower body fat percentage. Hormones also play a role - testosterone promotes muscle growth and metabolic rate. Example: A 30-year-old, 70kg, 170cm tall person has BMR of approximately 1,680 calories (male) vs 1,520 calories (female) - a 160 calorie difference. This gap widens with age as men tend to maintain muscle mass better. During menstruation, women's BMR can temporarily increase by 5-10% (100-150 calories). Pregnancy and breastfeeding significantly increase calorie needs by 300-500 calories daily. These differences mean women generally need fewer calories for maintenance and weight loss.
What is the difference between BMR and TDEE?
BMR (Basal Metabolic Rate) is calories burned at complete rest for basic body functions. TDEE (Total Daily Energy Expenditure) is total calories burned including all activity. TDEE = BMR + activity calories + thermic effect of food (digestion). Example: Woman with BMR of 1,400 calories who exercises moderately has TDEE of ~2,170 calories (1,400 × 1.55). The difference (770 calories) comes from: physical activity (exercise and daily movement), digestion (10% of calories consumed), and NEAT (non-exercise activity thermogenesis - fidgeting, standing, etc.). Use BMR to understand baseline metabolism. Use TDEE to set calorie targets for weight goals. Common mistake: eating at BMR level while exercising heavily - this creates too large a deficit and can damage metabolism.
How accurate are BMR calculators?
BMR calculators using Mifflin-St Jeor or Harris-Benedict formulas have ~10-20% error margin. They're based on population averages and may not account for: individual metabolism variations, thyroid function, previous dieting history (metabolic adaptation), body composition differences, genetics, and medication effects. For most people, calculators provide a good starting point. More accurate methods: DEXA scan with RMR measurement (±5% accuracy, costs £50-150), metabolic testing in lab (gold standard, ±3% accuracy, costs £100-300), or indirect calorimetry. Use calculator BMR as starting point, track weight and energy levels for 2-4 weeks, then adjust calories based on real-world results. If not losing/gaining weight as expected, adjust by 100-200 calories and reassess.