How your daily burn is estimated
BMR — basal metabolic rate — is the energy your body uses at complete rest to run its organs. This calculator uses the Mifflin-St Jeor equation, which has held up better against measured metabolic rates than the older Harris-Benedict formula.
TDEE multiplies BMR by an activity factor covering exercise, daily movement and digestion. That multiplier is the crude part of the estimate: most people overrate their activity level, so if your weight is not moving as expected, dropping one band is usually more accurate than distrusting the maths.
Using the deficit numbers sensibly
A kilogram of body fat holds roughly 7,700 kcal, so a 550 kcal daily deficit predicts about half a kilogram a week. Real loss is messier — water weight, glycogen and hormonal shifts move the scale far more than fat does over any given fortnight.
Metabolism also adapts. Sustained deficits reduce both BMR and unconscious movement, so the deficit that worked in month one is smaller in real terms by month four. Recalculate as your weight changes rather than holding the original target.
Frequently asked questions
Why is my calculated TDEE different from my fitness tracker? +
Trackers estimate active calories from heart rate and movement, which is a different method with its own error. Both are estimates; trust the trend in your weight over three or four weeks more than either number.
Is a very low calorie target safe? +
Going far below your BMR for extended periods risks muscle loss and nutritional deficiency. Aggressive deficits should be medically supervised.