Calorie Deficit Calculator
Tell the calculator where you are, where you want to be and how long you want to take. It works out the daily deficit required — and flags it when the answer is unrealistic.
The arithmetic behind it
A kilogram of body fat stores approximately 7,700 kcal (3,500 kcal per pound). The calculation is therefore:
The 7,700 figure comes from the energy density of adipose tissue, which is about 87 % lipid at 9 kcal per gram. It is a good approximation, but treat it as such: early weight loss includes a large water component as glycogen stores empty, which is why the first week often shows a much bigger drop than the maths predicts, and why week three can look like nothing is happening.
What rate is realistic
The commonly cited safe range is 0.5–1.0 kg per week, but a better guide scales with body size: 0.5 to 1.0 % of body weight per week. For a 120 kg person that is up to 1.2 kg weekly; for a 60 kg person, no more than 0.6 kg.
The reason is lean tissue. The leaner you already are, the more of any aggressive deficit comes from muscle rather than fat. Someone at 35 % body fat can tolerate a large deficit with little lean loss; someone at 12 % cannot.
Calorie floors. Intakes below roughly 1,500 kcal for men and 1,200 kcal for women make it genuinely difficult to hit micronutrient requirements, and are associated with poor adherence. If the calculator suggests going below that, the right fix is a longer timeline, not a smaller plate.
Why progress stalls
A deficit that worked in week one will not work in week ten, for four compounding reasons:
- You are lighter. A smaller body costs less to run. Ten kilograms down, maintenance may have fallen by 150–250 kcal on arithmetic alone.
- Adaptive thermogenesis. Metabolic rate falls by more than the weight change predicts — often an extra 10 % — as the body defends its stores.
- NEAT declines. Spontaneous daily movement drops without you noticing. This is frequently the largest single factor.
- Portion creep. Tracking accuracy erodes over weeks. Untracked oils, drinks and tastes add up quickly.
Recalculate after every 5 kg lost, and if the weekly average has genuinely not moved for three weeks, either take another 10 % off intake or add deliberate movement — not both at once, or you will not know which worked.
Frequently asked questions
Can I lose fat faster with a bigger deficit?
You will lose weight faster, but a declining share of it will be fat. Above roughly 25 % below maintenance, lean tissue loss accelerates, training performance drops and adherence typically collapses within weeks. The deficit you actually complete beats the one that looks best on paper.
Do I need to eat back exercise calories?
No, if you selected an activity level above sedentary — that exercise is already built into your TDEE. Eating it back again double counts it and erases the deficit.
Why did I gain weight this week despite a deficit?
Almost always water and gut contents. A salty meal, a new training stimulus, poor sleep or the luteal phase of the menstrual cycle can each add 1–2 kg of water overnight. Fat gain requires a genuine surplus; a single week of scale noise is not evidence of one. Judge by weekly averages.
Is this calculator suitable for everyone?
No. It is not appropriate during pregnancy or breastfeeding, for anyone under 18, for people with a history of disordered eating, or for anyone managing a condition where weight change needs supervision. In those situations, speak to a clinician rather than a calculator.