Daily energy change needed to hit a goal weight in a chosen timeframe.
Source: Wishnofsky M. Am J Clin Nutr 1958 (1 kg fat ≈ 7,700 kcal); safe-rate thresholds adapted from Helms et al., JISSN 2014.
What does a calorie deficit mean?
A calorie deficit occurs when you consume fewer calories than your body expends. Because the body needs energy to function, it must obtain it from stored reserves when dietary intake is insufficient. Fat tissue, which contains approximately 7,700 kcal per kilogram (from Wishnofsky's classic 1958 analysis in the American Journal of Clinical Nutrition), is the primary fuel reservoir mobilised during a sustained deficit.
This calculator divides the total energy difference needed to change weight (goal weight change × 7,700 kcal/kg) by the number of days in your chosen timeline, giving the daily calorie adjustment required. A positive result means a deficit (eat less than maintenance), and a negative result means a surplus (eat more than maintenance, for weight gain).
The 7,700 kcal/kg rule is a simplification. Real-world weight change also involves changes in water retention, muscle mass, and metabolic adaptation. The actual rate of weight loss often slows over time as the body adapts. Despite these limitations, the model is a useful planning tool when combined with regular reassessment every 4–6 weeks.
Safe rates of weight loss from Helms et al. (2014) are used as guardrails: more than 1–1.5% of body weight per week increases the risk of losing muscle mass rather than fat. For most people, a deficit of 300–600 kcal/day (0.3–0.6 kg/week) achieves sustainable fat loss with minimal muscle loss when combined with adequate protein intake.
Rate of change guidance
| Rate | Daily deficit | Suitability |
|---|---|---|
| Slow (0.25 kg/week) | ~270 kcal/day | Minimal muscle loss; sustainable long-term |
| Moderate (0.5 kg/week) | ~550 kcal/day | Practical for most adults; well tolerated |
| Aggressive (1 kg/week) | ~1,100 kcal/day | Requires high-protein diet; muscle loss risk |
| Very aggressive (>1 kg/week) | >1,100 kcal/day | Not recommended without medical supervision |
When should you see a doctor?
Before starting any significant caloric restriction, consult a doctor if you have a history of eating disorders, diabetes, heart disease, or take medications affected by body weight (such as insulin, some blood pressure drugs, and metformin). Very-low-calorie diets (below 800 kcal/day) require medical supervision and typically include dietitian support for meal planning and micronutrient supplementation.