BMR Calculator

About the BMR Calculator

A BMR calculator (Basal Metabolic Rate calculator) measures the minimum calories your body needs to sustain basic life functions while at complete rest — the energy your vital organs consume just to keep you alive: breathing, heartbeat, temperature regulation, cell repair, brain activity, and hormone production. BMR is the foundation of all calorie calculations in nutrition. It typically represents 60-75% of your total daily energy expenditure and defines the absolute floor below which you should never eat for extended periods without medical supervision. Our free BMR calculator uses the Mifflin-St Jeor equation, validated as the most accurate BMR formula for the general adult population in peer-reviewed research, and also computes the Harris-Benedict and Katch-McArdle formulas for comparison. BMR is closely related to TDEE (Total Daily Energy Expenditure) — your TDEE is your BMR multiplied by your activity level. Understanding your BMR is the first step toward any evidence-based nutrition plan because without this baseline, every calorie target is a guess. The calculator works in both metric (kg/cm) and imperial (lbs/feet/inches) and includes an explanation of how age, sex, height, weight, and body composition each contribute to your individual BMR.

How It Works

Mifflin-St Jeor (recommended, most accurate for general population): Men: BMR = (10 x weight kg) + (6.25 x height cm) - (5 x age) + 5. Women: BMR = (10 x weight kg) + (6.25 x height cm) - (5 x age) - 161. Example: 35-year-old woman, 65 kg, 163 cm. BMR = (650) + (1018.75) - (175) - 161 = 1,332.75 calories/day at rest. Harris-Benedict (original 1919, revised 1984 — widely used but less accurate for overweight individuals): Men: 88.362 + (13.397 x kg) + (4.799 x cm) - (5.677 x age). Women: 447.593 + (9.247 x kg) + (3.098 x cm) - (4.330 x age). Katch-McArdle (most accurate for lean individuals if body fat % is known): BMR = 370 + (21.6 x Lean Body Mass in kg). Since this woman's BMR is 1,333 calories, she burns that amount simply existing — before any movement, exercise, or digestion. Everything above that is attributable to activity.

Tips & Best Practices

• ✓Never eat below your BMR for extended periods. Severe restriction below BMR forces muscle catabolism for energy, triggers hormonal suppression of metabolism, and creates the metabolic adaptation that makes future dieting progressively harder.
• ✓BMR declines approximately 1-2% per decade after age 30, primarily due to age-related muscle loss (sarcopenia). Consistent resistance training is the most effective intervention to preserve and even increase BMR with age.
• ✓Two people of identical height and weight can have BMRs differing by 200-300 calories if their body compositions differ significantly — the person with more muscle mass burns meaningfully more at rest every single day.
• ✓Thermic Effect of Food (TEF) adds approximately 10% to your daily calorie burn above BMR — your body expends energy digesting, absorbing, and metabolising food. High-protein meals have TEF of 20-30%, making protein inherently metabolism-friendly.
• ✓Thyroid hormones (T3 and T4) are the primary regulators of BMR. Hypothyroidism reduces BMR by 10-30%; hyperthyroidism raises it. Unexplained weight changes that resist diet and exercise intervention warrant thyroid function testing.
• ✓Fever raises BMR: every 1 degree Celsius increase in body temperature raises metabolic rate approximately 10-13%, which is why illness causes unintentional weight loss despite reduced appetite.
• ✓Crash diet warning: 8 weeks of 25% caloric restriction can reduce BMR by an additional 8-10% beyond what is explained by weight loss alone — this "metabolic suppression" can persist for months after the diet ends, a key mechanism behind yo-yo weight cycling.
• ✓Clinical measurement of BMR uses indirect calorimetry: measuring oxygen consumed and carbon dioxide produced over a resting period. Calculator formulas estimate BMR with plus or minus 10% accuracy for most individuals — use tracked weight trends to calibrate.

Who Uses This Calculator

People starting a weight loss programme use BMR to establish the minimum calorie floor that protects muscle mass and metabolic function, preventing the restrictive dieting mistakes that cause long-term metabolic damage. Clinical registered dietitians use BMR as the medical foundation for calculating nutritional requirements in hospital patients, including tube feeding and parenteral nutrition prescriptions where precision directly affects patient outcomes. Personal trainers explain BMR to clients as the physiological reason why "starvation diets" damage long-term metabolism and why sustainable deficits produce better body composition outcomes than extreme restriction. Athletes in weight-class sports use BMR calculations to plan controlled weight cuts that minimise muscle and performance loss while reaching competition weight. People recovering from eating disorders use BMR as an objective, science-based starting point for rebuilding healthy eating patterns and understanding their body's actual energy needs. Researchers in metabolism, obesity medicine, and sports science use BMR measurement and estimation in study protocols investigating energy balance, metabolic adaptation, and the physiological responses to different dietary interventions.