Calorie deficitCaloric deficit · Energy deficit
Taking in less energy than you expend: the condition under which fat loss happens.
A calorie deficit is the state of taking in less energy than you spend. It is the condition under which stored fat is used, and every diet that has ever worked, whatever it was called, worked by producing one.
That is a statement about arithmetic rather than about method. Low carbohydrate, high protein, fasting windows, meal timing and food quality all influence how easy the deficit is to create and hold, and none of them replaces it.
The usual conversion rule is that a kilogram of body weight is worth about 7,700 kcal, so a deficit of 500 kcal a day predicts roughly half a kilogram a week. Treat it as a first estimate: the rule was derived in 1958 and consistently overstates long term loss, because expenditure falls as you get lighter and spontaneous movement quietly declines with it.
Size matters in both directions. Too small and the signal is lost in measurement noise for weeks. Too large and you lose more muscle, feel worse, and are more likely to abandon the whole thing. A deficit of roughly 15 to 25% below maintenance suits most people, with leaner people needing the gentler end.
Two habits protect what you keep. Eating enough protein and continuing to lift preserve muscle while fat comes off, which is the difference between finishing a diet lighter and finishing it smaller in the way you wanted.
Progress is not linear and should not be read daily. Water shifts from carbohydrate, salt, training and the menstrual cycle can hide a fortnight of real fat loss, so a weekly average is the honest measurement and the scale on any single morning is mostly noise.
In practice, estimate maintenance with our TDEE calculator, set the deficit with the calorie deficit calculator, then re-estimate every few kilograms, because the number you started with stops being true as you get smaller.