Calorie Deficit & Weight Loss Calculator
Estimate how much weight you will lose over a set number of weeks by entering your maintenance calories, target intake, and exercise burn. Perfect for planning a structured fat-loss phase or evaluating whether a deficit is realistic.
About this calculator
Weight loss fundamentally depends on creating a sustained calorie deficit — consuming fewer calories than your body expends. The widely used rule of thumb is that approximately 7,700 kcal of deficit equates to 1 kg of body fat lost (based on the energy content of adipose tissue). The formula used here is: Weight Lost (kg) = ((maintenance_calories − target_calories + exercise_calories) × 7 × weeks) / 7,700. The daily deficit is multiplied by 7 to get a weekly deficit, then by the number of weeks to find the total deficit, which is finally divided by 7,700 to convert to kilograms. Exercise calories burned are added to the deficit because they represent additional energy expenditure beyond your maintenance baseline. In practice, metabolic adaptation means results may slow over time, but this formula provides a solid planning estimate.
How to use
Suppose your maintenance calories are 2,500 kcal/day, you plan to eat 2,000 kcal/day, and you burn an extra 300 kcal/day through exercise over 8 weeks. Step 1 — Daily deficit: (2,500 − 2,000 + 300) = 800 kcal/day. Step 2 — Total deficit: 800 × 7 × 8 = 44,800 kcal. Step 3 — Weight loss: 44,800 / 7,700 ≈ 5.8 kg over 8 weeks. This equates to roughly 0.73 kg per week, which falls within the safe and sustainable recommended range of 0.5–1 kg per week.
Frequently asked questions
How large should a calorie deficit be for safe and sustainable weight loss?
Most nutrition and health bodies recommend a deficit of 500–750 kcal/day as a sustainable target, which typically produces 0.5–0.75 kg of weight loss per week. Larger deficits (above 1,000 kcal/day) can accelerate fat loss but increase the risk of muscle loss, nutrient deficiencies, fatigue, and metabolic adaptation. Very low calorie diets (below 800 kcal/day) should only be followed under medical supervision. A moderate deficit paired with adequate protein intake and resistance training is generally the most effective strategy for losing fat while preserving muscle mass.
Why does weight loss slow down over time even when maintaining the same calorie deficit?
As you lose weight, your body requires fewer calories to function because there is less mass to sustain — your maintenance calorie level drops. Additionally, the body undergoes metabolic adaptation (sometimes called 'adaptive thermogenesis'), where it becomes more efficient and reduces non-exercise energy expenditure in response to prolonged calorie restriction. This means the same deficit that produced rapid loss initially will yield slower results over time. To counter this, you can periodically recalculate your maintenance calories as your weight changes and adjust your intake or increase exercise accordingly.
Does the 7,700 calories per kilogram of fat rule apply to everyone equally?
The 7,700 kcal/kg figure is a population-level average based on the caloric density of human adipose tissue, which contains roughly 87% fat at about 9 kcal/g. In practice, individual results vary because some weight loss comes from water and glycogen (particularly early on), not just fat. People with higher starting body fat percentages may see slightly different energy equivalents. Nonetheless, the 7,700 kcal rule remains a well-validated and practically useful planning estimate for most adults pursuing gradual, sustained fat loss.