Calorie Deficit Calculator how long it actually takes
Enter your body, your goal weight and how big a deficit you want. You get the intake to eat today, the rate of loss, and the week the goal arrives.
The Curve Bends
A lighter body burns less, so the projection slows down instead of running straight.
7,700 kcal per kg Is a Convention
A useful approximation, not a law of physics — the limitations section explains where it breaks.
What is a calorie deficit?
At a Glance
A calorie deficit is eating less energy than you spend, which forces the body to cover the difference from its own stores. This calculator turns a deficit into the two numbers that matter in practice: what to eat today, and how many weeks the goal will take.
Three steps, run again for every week of the projection rather than once at the start.
Step one: maintenance today. The Mifflin-St Jeor equation gives the basal rate, and the FAO/WHO/UNU activity multiplier turns it into total daily energy expenditure. For the default persona that is 2,533 kcal.
Step two: the deficit. A percentage rather than a flat number, so it scales with the body it is applied to:
intake = maintenance − (maintenance × deficit %)Step three: turn energy into weight. The familiar convention values body-fat tissue at about 7,700 kcal per kilogram — the metric form of "3,500 kcal per pound":
weekly kg = (daily deficit × 7) ÷ 7,700Then the loop repeats at the new, lighter weight. That is the part most calculators skip. According to Hall and colleagues in The Lancet, a static model overstates weight loss precisely because it ignores the fall in expenditure that comes with a smaller body. On the default figures, maintenance drops from 2,533 kcal at 90 kg to 2,410 by week 20 — and the projected curve visibly flattens.
These are the values the calculator loads by default.
Find maintenance at the starting weight
Mifflin-St Jeor for a 35-year-old man at 90 kg and 178 cm, times 1.375 for light activity: 2,533 kcal a day.
Apply the deficit
20 % of 2,533 = 507 kcal, so the intake target is 2,027 kcal a day.
Convert to weight
507 × 7 = 3,549 kcal a week, ÷ 7,700 = 0.46 kg in week one.
Recalculate at the new weight
By week 10 the projection is at 85.5 kg, maintenance has fallen to 2,471, and the intake target is 1,977 — the deficit shrinks along with the body.
Read the timeline
The goal of 80 kg arrives in week 23. At a 10 % deficit it would take 45 weeks; at 25 %, 18.
The timeline is a projection under fixed assumptions, not a schedule.
A bigger deficit is not proportionally faster. Going from 10 % to 20 % cuts the timeline from 45 weeks to 23 — nearly half. Going from 20 % to 25 % only takes it from 23 to 18, while making the diet meaningfully harder and costing more lean mass. The returns flatten as the difficulty rises.
0.5 to 1 % of body weight per week is the usual sustainable band. For a 90 kg person that is 0.45 to 0.9 kg. The default 20 % deficit lands right at the bottom of it, which is why it is a reasonable starting point rather than an aggressive one.
Expect the scale to disagree in the short run. Water, sodium, glycogen and gut contents move body weight by a kilogram or more within a day. Compare weekly averages, not individual mornings.
The curve bending is the real lesson. Every kilogram lost lowers maintenance, so the same intake produces a smaller deficit over time. When progress stalls, the usual reason is not a broken metabolism — it is that the intake which once created a deficit has become the new maintenance.
For the maintenance figure on its own, use the TDEE Calculator. To split the daily intake into protein, fat and carbohydrate, use the Macro Calculator.
What this projection cannot promise
This calculator is for informational purposes and is not medical advice or a diagnosis.
- 7,700 kcal per kilogram is a convention. It assumes the loss is pure fat. Real loss includes water and some lean tissue, especially early on and in aggressive deficits.
- Adaptive changes are not modelled. Non-exercise movement tends to fall during a deficit, and hormonal adaptation reduces expenditure further than body weight alone predicts.
- The starting estimate carries roughly 10 % of error. That flows straight into the timeline.
- Very low intakes need supervision. Below roughly 1,200 kcal for women and 1,500 for men, discuss the plan with a doctor or registered dietitian.
- It projects weight, not health. Weight is one crude marker; body composition, strength, blood markers and how you feel are not in this model.
- A goal below a healthy weight is out of scope. Check your goal against a healthy BMI range with the BMI Calculator.
If you are managing a health condition, pregnant, or have a history of disordered eating, talk to a doctor before starting a deficit.