BMR Calculator

This calculator uses two validated scientific formulas to estimate your resting metabolic rate. The Mifflin-St Jeor equation is considered the gold standard for accuracy in most adults, while Harris-Benedict provides a useful comparison point.

Example Calculation

For a 28-year-old female, 65 kg, 165 cm:

• BMR = 10(65) + 6.25(165) − 5(28) − 161
• BMR = 650 + 1,031.25 − 140 − 161
• BMR = 1,380 calories/day

This means her body burns approximately 1,380 calories just to maintain basic functions at complete rest.

Tips for Using Your BMR

• Never eat below your BMR for extended periods
• Multiply BMR by your activity factor to get TDEE (total daily calories)
• Muscle mass increases BMR — prioritize strength training
• Crash diets can reduce BMR by 15-20% through metabolic adaptation
• Sleep deprivation reduces BMR — aim for 7-9 hours nightly

Limitations

BMR equations provide useful estimates but are not clinically precise:

• Predictive equations match measured BMR within 10% for approximately 80% of healthy adults. The remaining 20% may see larger deviations due to thyroid function, body composition, or genetic factors not captured by height/weight/age inputs
• Does not account for: body composition (muscle vs fat), genetics, thyroid function, medications
• Less accurate for: very muscular individuals (underestimates), obese individuals (overestimates), elderly (overestimates due to muscle loss)
• Harris-Benedict tends to overestimate by 5-15% compared to Mifflin-St Jeor
• Cannot replace indirect calorimetry for clinical applications (e.g., ICU nutrition planning)
• Assumes a normal, healthy resting state — illness, fever, stress, and recovery all increase actual BMR

Common Mistakes

• Confusing BMR with TDEE — BMR is only your resting burn, not what you should eat
• Using BMR as a calorie target (you need to eat MORE than BMR for basic daily function)
• Not accounting for the thermic effect of food (~10% of total intake)
• Assuming BMR is fixed — it decreases with age (~1-2% per decade after 20) and with weight loss
• Using an old weight — recalculate when weight changes by 5+ kg
• Choosing the wrong formula — Mifflin-St Jeor is recommended over Harris-Benedict for accuracy

What Happens to BMR When You Diet

Prolonged calorie restriction triggers metabolic adaptation (sometimes called "adaptive thermogenesis"), reducing your BMR by 5-15% beyond what weight loss alone would predict. This means a person who has dieted down to 70 kg may have a BMR 100-200 calories lower than someone who naturally weighs 70 kg. This adaptation is your body's survival mechanism — it becomes more efficient at conserving energy during perceived famine. The reduction is not permanent but can persist for months after returning to maintenance calories. Structured diet breaks (1-2 weeks at maintenance every 8-12 weeks) can partially mitigate this effect.

How to Measure Your True BMR

The clinical gold standard for measuring BMR is indirect calorimetry, which analyzes oxygen consumption and CO₂ production while you rest in a controlled environment. This test costs $100-300 at metabolic testing clinics and provides accuracy within 2-3%. Consumer fitness trackers (wrist-based heart rate monitors) estimate BMR using algorithms based on your profile data, but their accuracy varies widely — studies show errors of 10-40% for individual measurements. If precision matters for your goals, a single indirect calorimetry test gives you a reliable anchor point that you can then track relative changes from using a wearable device.

Alternative Metrics

• Indirect calorimetry (direct measurement of oxygen consumption — clinical gold standard)
• Katch-McArdle formula: BMR = 370 + (21.6 × lean body mass in kg) — requires body fat % measurement
• Cunningham equation: BMR = 500 + (22 × lean body mass) — designed for athletes
• Resting Metabolic Rate (RMR) — slightly higher than BMR as it includes minimal activity