Calorie Deficit Calculator

Evidence-Based Fat Loss

The 7,700 kcal Rule

One kilogram of body fat stores approximately 7,700 kcal. A deficit of 500 kcal/day creates a weekly deficit of 3,500 kcal, theoretically losing 0.45 kg per week. In practice, actual fat loss is slightly variable due to water retention, muscle preservation, and metabolic adaptation — but over 4–6 weeks, the maths holds closely enough for reliable planning.

Why Protein Matters Most in a Deficit

In a calorie deficit, your body can break down muscle tissue for energy. High protein intake (1.8–2.4g per kg of body weight) is the primary defence against muscle loss. Studies show that during a 500 kcal deficit, high-protein dieters lose significantly less lean mass than low-protein dieters at the same calorie level. Protein also has the highest thermic effect (25–30% of calories are burned in digestion) and the strongest satiety effect.

Metabolic Adaptation — Why Deficits Shrink

As you lose weight, your body adapts: BMR decreases (your smaller body needs fewer calories), activity-related calorie burn decreases (less mass to move), and hormonal changes (leptin, thyroid) reduce metabolic rate further. Total adaptation can reduce TDEE by 200–400 kcal below what weight-based formulas predict. This is why recalculating TDEE every 4–5 kg of loss is essential.

Diet Breaks and Maintenance Periods

Planned breaks of 1–2 weeks at maintenance calories help reset leptin, reduce cortisol, and improve adherence. Research on diet breaks suggests they preserve metabolic rate better than continuous deficits. Practically: a 12-week cut followed by a 2-week maintenance phase before continuing is more effective than 14 weeks of continuous restriction.