About this tool
Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest just to keep vital functions running — breathing, circulation, cell repair. This calculator uses the Mifflin-St Jeor equation, published in 1990 and repeatedly validated since as the most accurate predictive formula for the general population (more accurate than the older 1919 Harris-Benedict equation, which tends to overestimate). From your sex, weight, height and age it computes BMR, then multiplies by an activity factor to estimate your Total Daily Energy Expenditure (TDEE) — the calories you actually burn in a typical day, including movement and exercise.
TDEE is the number to build a nutrition plan around: eating at TDEE maintains your current weight, a consistent daily deficit produces weight loss, and a consistent surplus produces weight gain. The table below shows five common targets built on the standard rule of thumb that roughly 3,500 kcal equals one pound of body fat — so a 250 kcal/day deficit or surplus works out to about half a pound a week, and 500 kcal/day to about a pound a week. These are population-level estimates, not a personalized prescription; individual metabolism, body composition, medical conditions and medications all shift real-world energy needs, so treat the numbers as a starting point to adjust based on how your own weight actually responds, and check with a professional for anything beyond casual tracking.
Frequently asked questions
Why does this calculator use the Mifflin-St Jeor equation instead of Harris-Benedict?
The original Harris-Benedict equation dates from 1919 and was revised in 1984, but multiple validation studies since then have found the newer Mifflin-St Jeor equation (1990) more accurately predicts measured resting metabolic rate in modern populations, which is why it's now the equation most commonly recommended by dietetics professional bodies for healthy adults.
How reliable are the "add/subtract 250 or 500 calories" weight-change estimates?
The "3,500 kcal ≈ 1 lb of fat" rule of thumb behind these targets is a useful starting approximation, but real-world weight change is not perfectly linear — the body's TDEE actually shifts as weight changes (a smaller body burns somewhat fewer calories at rest), and individual metabolic adaptation varies. Treat these numbers as a reasonable starting point to adjust from based on your own real-world results over several weeks, not an exact prediction.