How this calculator works
This calculator turns your total daily energy expenditure (TDEE) and a chosen daily deficit into calories to eat, an estimated weekly weight change, and roughly how many weeks it will take to reach a goal weight. If you don't already know your TDEE, use the BMR & TDEE calculator to estimate it from your height, weight, age and activity level first.
The weekly-loss estimate relies on the widely cited convention that roughly 7,700 kcal correspond to 1 kg of body fat (about 3,500 kcal per pound). This is an approximation, not a precise law of physiology — it assumes a static body that doesn't adapt, when in reality metabolism and appetite both shift as you lose weight, so real-world progress usually slows compared to a straight-line projection over many weeks.
This is a planning estimate, not medical or nutrition advice. Aggressive or very low targets carry real risks, so check with a doctor or registered dietitian before making a significant, sustained cut to how much you eat.
The formulas
Calories to eat = TDEE − daily deficitWeekly weight change ≈ (daily deficit × 7) ÷ 7,700 kg, or ÷ 3,500 lbWeeks to goal ≈ (current weight − goal weight) ÷ weekly weight change7,700 kcal/kg (≈3,500 kcal/lb) is a long-standing approximation (Wishnofsky, 1958) for the energy density of body fat tissue. It is a useful planning estimate, not an exact conversion — see the sources below for why it tends to overestimate long-term loss.
Worked example: TDEE 2,500 kcal, 500 kcal/day deficit
- Calories to eat: 2,500 − 500 = 2,000 kcal/day.
- Weekly change: (500 × 7) ÷ 7,700 ≈ 0.45 kg/week (about 1 lb/week).
- Going from 90 kg to a goal of 80 kg is a 10 kg difference.
- Weeks to goal: 10 ÷ 0.4545 ≈ 22 weeks.
Frequently asked questions
Why did I lose less than the calculator predicted?
The 7,700 kcal/kg convention assumes your TDEE stays constant, but TDEE actually falls as you lose weight (a smaller body burns less) and often further from metabolic adaptation. Research specifically examining this rule has found it tends to overpredict real-world weight loss over many weeks.
Why does the calculator warn me below 1,200 calories?
1,200 kcal/day is a commonly cited floor for adult diets, below which it becomes harder to meet basic nutrient needs and more likely that lean mass, energy and adherence all suffer. If your target falls below it, consider a smaller deficit or talk to a doctor or registered dietitian.
Which deficit size should I choose?
Smaller deficits (250-500 kcal/day) are easier to sustain and preserve more muscle; larger ones (750-1,000 kcal/day) move faster but are harder to stick with and increase the risk of muscle loss and rebound hunger. There is no single right answer — sustainability matters more than speed.
Does the type of food I eat matter if the calorie deficit is the same?
For weight change, the deficit itself is what drives the scale. Food choices still matter for hunger control, muscle preservation (protein) and long-term health, which is why calories and macronutrient balance are usually planned together.
What happens if I set a deficit that's larger than my TDEE?
The 'calories to eat' figure would drop to zero or below, which isn't a workable target — you can't eat negative calories, and even reaching zero would be far below the 1,200 kcal/day floor this calculator already warns about. If your chosen deficit produces an unrealistic or very low number, pick a smaller one, ideally in the 250-500 kcal/day sustainable range described above.