Calorie Deficit Calculator

A calorie deficit is the single mechanism that makes fat loss happen. Eat fewer calories than you burn and the body draws on stored energy; eat more and it stores the surplus. Everything else — keto, intermittent fasting, low-fat, low-carb — works by making that deficit easier to hold, not by breaking the arithmetic.

The energy in a kilogram of body fat is roughly 7,700 calories, the same figure as about 3,500 calories per pound. That is why a daily deficit of 500 calories produces a theoretical loss of about half a kilogram a week, and a deficit of 1,000 calories produces about a kilogram. These are the classic numbers, and they are a reasonable planning baseline, with one important caveat explained below.

Set the deficit against your total daily energy expenditure, not against some fixed number like 1,200 calories. A large, active man might maintain on 3,000 calories and feel fine on 2,400. A small, sedentary woman might maintain on 1,800 and find 1,300 genuinely hard. The same 500-calorie deficit means very different things depending on the starting point, which is why calculating your own TDEE first matters.

The caveat is metabolic adaptation. As you lose weight, your maintenance requirement falls — partly because there is less of you to carry around, and partly because the body trims its own energy use in response to a sustained deficit. In practice this means weight loss slows over time even when the deficit is held constant. Recalculate every four to six weeks using your new weight, rather than trusting the first number forever. And keep protein high and lift weights while you diet: it protects muscle, which protects the metabolic rate you are trying not to lose.

This calculator returns estimates for general information only. It is not medical advice, diagnosis or treatment, and it cannot replace an assessment by a qualified health professional. Individual results vary, and figures such as calorie needs, body-fat percentage and heart-rate zones are approximations based on population averages rather than measurements of you.