Magic Number Calculator — Days to Goal Weight

Formula

How this is calculated

The central idea is that burning (or creating a deficit of) roughly 7,700 kilocalories results in approximately 1 kilogram of fat loss. This is the Wishnofsky rule of thumb, derived from the energy content of human adipose tissue (~38 MJ/kg ≈ 9.4 kcal/g for pure fat, adjusted for the water and protein content of real body-fat tissue). It is a widely used estimate for planning purposes.

To find your "magic number" of days: first calculate the total energy that must be created in deficit — the difference between current and goal weight in kilograms multiplied by 7,700 kcal. Then divide that by the daily calorie deficit (how many fewer calories you eat than you burn each day). For example, to lose 10 kg at a 500 kcal/day deficit: (10 × 7,700) / 500 = 154 days, or about 22 weeks.

Important limitations: the 7,700 kcal/kg rule assumes all lost weight is fat, but real weight loss also involves water, glycogen, and some lean mass — especially at first. As you lose weight, your total daily energy expenditure decreases (metabolic adaptation), so the deficit naturally shrinks over time and progress slows. The formula gives a useful planning target rather than a precise guarantee. A sustained deficit of 300–500 kcal/day is generally recommended as safe; deficits above 1,000 kcal/day should only be undertaken with medical supervision.