Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest over 24 hours — the energy cost of keeping you alive while you do absolutely nothing. Breathing, circulating blood, maintaining body temperature, repairing cells, and running your brain and organs all draw power continuously, and for most people this idle consumption accounts for 60 to 75 percent of all calories burned in a day. Knowing your BMR matters because every calorie plan is built on top of it: your total daily expenditure is your BMR multiplied by an activity factor, so an accurate BMR is the foundation of any honest weight-loss, weight-gain, or maintenance target.
The most widely validated BMR formula for healthy adults is the Mifflin-St Jeor equation, published in 1990 and still the default in clinical nutrition. For men: BMR = (10 x weight in kg) + (6.25 x height in cm) - (5 x age in years) + 5. For women: BMR = (10 x weight in kg) + (6.25 x height in cm) - (5 x age in years) - 161. The structure tells the physiological story: weight and height raise the number because a larger body costs more to run, age lowers it because muscle mass and metabolic rate decline over time, and the sex constant reflects average differences in body composition. Typical results fall between roughly 1,200 and 2,000 kcal per day. Older formulas like Harris-Benedict (1919, revised 1984) tend to overestimate for today's populations, which is why Mifflin-St Jeor is preferred.
A formula cannot see your muscle mass, thyroid function, or medical history, so treat the result as a strong estimate rather than a lab measurement. Two people with identical height, weight, age, and sex can differ by 10 percent or more in true resting expenditure because muscle burns more than fat at rest. To turn BMR into a daily calorie target, multiply by an activity factor: 1.2 for sedentary, 1.375 for light activity, 1.55 for moderate, 1.725 for very active, and 1.9 for extremely active lifestyles. This calculator is informational only — it cannot diagnose metabolic conditions, and anyone with a thyroid disorder, on regular medication, pregnant, or managing a medical condition should have energy needs assessed by a doctor or registered dietitian.