BMR Calculator

About the BMR Calculator

A BMR calculator (Basal Metabolic Rate calculator) measures the minimum number of calories your body needs to sustain basic life functions while at complete rest — the energy your vital organs consume simply to keep you alive: breathing, heartbeat, temperature regulation, cell repair, brain activity, hormone production, and kidney filtration. BMR is the foundation of all calorie calculations in nutrition science. It typically represents 60 to 75 percent of your total daily energy expenditure and defines the absolute physiological 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 across multiple peer-reviewed studies, and also computes the Harris-Benedict and Katch-McArdle formulas for comparison purposes. BMR is closely related to TDEE (Total Daily Energy Expenditure) — your TDEE is simply your BMR multiplied by an activity factor that accounts for everything you do above complete rest. Understanding your BMR is the first step toward any evidence-based nutrition plan because without this baseline, every calorie target is an educated guess at best. 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. Several factors influence BMR that go beyond height and weight. Thyroid function is one of the primary regulators — hypothyroidism can reduce BMR by 10 to 30 percent, while hyperthyroidism can raise it significantly, sometimes explaining unexplained weight changes that resist dietary intervention. Body composition matters enormously: muscle tissue is metabolically active and burns approximately 6 to 10 calories per pound per day at rest, while fat tissue burns only about 2 calories per pound per day. This is why two people of identical height and weight but different body compositions can have BMRs that differ by 200 to 300 calories per day. As people age, BMR typically declines by 1 to 2 percent per decade after age 30, primarily driven by sarcopenia — the gradual age-related loss of muscle mass that begins in the mid-30s and accelerates after 60 if not addressed through resistance training and adequate protein intake. BMR information is used in clinical settings for hospital nutrition support, medication dosing, and the management of conditions affecting metabolism. Always use your BMR result as a starting point for broader nutritional planning, and consult a registered dietitian for personalised guidance.

How It Works

The Mifflin-St Jeor equation is recommended as the most accurate for the general population. Men: BMR = (10 x weight kg) + (6.25 x height cm) minus (5 x age) + 5. Women: BMR = (10 x weight kg) + (6.25 x height cm) minus (5 x age) minus 161. Example: a 35-year-old woman weighing 65 kg at 163 cm. BMR = 650 + 1018.75 minus 175 minus 161 = 1,332.75 calories per day at rest. The Harris-Benedict equation (original 1919, revised 1984) is also widely used but less accurate for overweight individuals. Men: 88.362 + (13.397 x kg) + (4.799 x cm) minus (5.677 x age). Women: 447.593 + (9.247 x kg) + (3.098 x cm) minus (4.330 x age). The Katch-McArdle formula is the most accurate for lean individuals when body fat percentage is known: BMR = 370 + (21.6 x Lean Body Mass in kg). For this example woman with a BMR of 1,333 calories, she burns that amount simply existing before any movement, exercise, or digestion. Everything above BMR is attributable to physical activity, the thermic effect of food, and exercise. Clinical measurement of true BMR uses indirect calorimetry — measuring oxygen consumed and carbon dioxide produced over a resting period in a controlled environment. Calculator formulas estimate BMR with plus or minus 10 percent accuracy for most individuals.

Tips & Best Practices

• ✓Never eat below your BMR for extended periods. Severe restriction below BMR forces the body into muscle catabolism for energy, triggers hormonal suppression of metabolic rate, and creates the metabolic adaptation that makes future dieting progressively harder — a pattern studied extensively by the National Institutes of Health.
• ✓BMR declines approximately 1 to 2 percent per decade after age 30, primarily due to age-related muscle loss known as sarcopenia. Consistent resistance training is the most effective intervention to slow and even partially reverse BMR decline with age, making it one of the best long-term investments in metabolic health.
• ✓Two people of identical height and weight can have BMRs differing by 200 to 300 calories per day if their body compositions differ significantly. The person with more muscle mass burns meaningfully more at rest every single day — demonstrating why body recomposition through resistance training produces long-term metabolic advantages.
• ✓Thermic Effect of Food adds approximately 8 to 10 percent to your daily calorie burn above BMR — your body expends energy digesting, absorbing, and metabolising food. High-protein meals have the highest TEF at 20 to 30 percent, making protein inherently the most metabolism-friendly macronutrient per calorie consumed.
• ✓Thyroid hormones T3 and T4 are primary regulators of BMR. Hypothyroidism reduces BMR by 10 to 30 percent while hyperthyroidism raises it significantly. Unexplained weight changes that resist diet and exercise intervention warrant thyroid function testing with TSH, free T3, and free T4 measurements.
• ✓Fever raises BMR: every 1 degree Celsius increase in body temperature raises metabolic rate approximately 10 to 13 percent, which is why serious illness causes unintentional weight loss despite reduced appetite. This thermal regulation mechanism requires substantial energy to sustain.
• ✓Crash diet warning: 8 weeks of 25 percent caloric restriction can reduce BMR by an additional 8 to 10 percent beyond what is explained by weight loss alone. This metabolic suppression, studied in contestants of The Biggest Loser, can persist for months or years after the diet ends — a key mechanism behind weight regain after rapid loss.
• ✓Clinical measurement of BMR uses indirect calorimetry: measuring oxygen consumed and carbon dioxide produced over a resting period in a fasted, thermoneutral state. Calculator formulas estimate BMR with plus or minus 10 percent accuracy for most people — use tracked weight trends over 2 to 3 weeks to calibrate your individual TDEE from real-world data.

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 severe restriction mistakes that cause long-term metabolic damage and the yo-yo weight cycling pattern. Clinical registered dietitians use BMR as the medical foundation for calculating nutritional requirements in hospital patients, including tube feeding and parenteral nutrition prescriptions where caloric precision directly affects patient outcomes and recovery. Personal trainers explain BMR to clients as the physiological reason why starvation diets damage long-term metabolism and why sustainable, moderate deficits produce better body composition outcomes than extreme restriction. Athletes in weight-class sports such as boxing, wrestling, powerlifting, and martial arts use BMR calculations to plan controlled weight cuts that minimise muscle and performance loss while meeting competition weight requirements. 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 — replacing the distorted internal perceptions common in disordered eating with real physiological data. Researchers in metabolism, obesity medicine, and sports science use BMR measurement and estimation in study protocols investigating energy balance, metabolic adaptation, and physiological responses to dietary interventions. Healthcare providers treating patients with thyroid disorders use BMR calculations to monitor the metabolic consequences of hormonal changes and assess the effectiveness of thyroid hormone treatment.