How long does losing weight really take?
Calorie deficit, the body-fat rule of thumb, and realistic expectations
The foundation of any weight loss is a calorie deficit: over a period of time, you need to consume less energy than your body uses. As a rule of thumb, 1 kg of body fat corresponds to roughly 7,700 kcal. With a daily deficit of 500 kcal, this works out to a weight loss of about 0.45 kg per week (500 × 7 ÷ 7,700).
Our calculator first determines your total daily energy expenditure (TDEE) using the Mifflin-St Jeor formula (as in the Calorie Needs Calculator) and subtracts your chosen calorie deficit to calculate your recommended daily calorie intake. The estimated number of days follows from the total energy deficit to reach your goal weight (weight difference × 7,700 kcal).
Healthy pace: Professional bodies recommend a pace of no more than 0.5 to 1 kg per week. That corresponds to a daily deficit of roughly 300 to 600 kcal (up to a maximum of about 1,100 kcal for the 1 kg upper limit). If you choose a significantly higher deficit, the calculator shows a warning: overly aggressive diets increase the risk of muscle loss, nutrient deficiencies, concentration problems, and rebound weight gain after the diet ends.
Important: the formula is a static model. In practice, energy expenditure decreases as body weight drops (due to less muscle mass and an adjusted resting metabolic rate, among other factors), so actual weight loss often slows over the course of a diet. The calculated duration is therefore a guide, not an exact prediction.