Calorie Deficit Calculator

A calorie deficit is simply eating less than you burn. It is the mechanism behind every diet that works, whatever else the diet claims to be doing. What this page answers is a narrow question: how big does that gap have to be to get from your current weight to a target weight in the time you have?

What this calculator does and does not do

It works backwards from a goal. Give it a starting weight, a target and a number of weeks, and it tells you the daily deficit that arithmetic requires.

It does not know your maintenance calories, because it does not ask for your age, height, sex or activity level. So it gives you the size of the gap, not the number to eat. For that, take the deficit here and subtract it from your TDEE, which the calorie calculator works out.

The formulas actually used here:

Total energy to lose = kilograms to lose × 7,700 kcal
Daily deficit = total energy ÷ (weeks × 7)
Weekly rate = kilograms to lose ÷ weeks

That 7,700 figure is the energy in a kilogram of body fat, and it is the metric twin of the familiar “3,500 calories per pound”. Both are approximations of the same thing: 7,700 kcal/kg works out to about 3,490 per pound.

How big a deficit is reasonable

A better way to think about the ceiling is as a percentage of bodyweight: up to about 1% a week is the usual guidance, and the leaner you already are, the lower it should be. A 120 kg person losing 1 kg a week is doing something quite different from a 60 kg person losing the same.

Worked example:

Current weight 85 kg, target 75 kg, timeline 20 weeks.

• To lose: 85 − 75 = 10 kg
• Total energy: 10 × 7,700 = 77,000 kcal
• Daily deficit: 77,000 ÷ (20 × 7) = 77,000 ÷ 140 = 550 calories a day
• Weekly rate: 10 ÷ 20 = 0.5 kg a week, which is 0.59% of bodyweight

If that person’s TDEE turns out to be 2,600, they eat about 2,050.

Why it will probably take longer than the number says

The 7,700 rule assumes your expenditure holds still while you shrink, and it does not. A lighter body costs less to move and less to maintain, so the same intake represents a smaller deficit each month. Appetite pushes back too, and non-exercise movement quietly falls.

Kevin Hall’s group at the US National Institutes of Health put numbers on this in 2011 (The Lancet). For a typical adult, a lasting cut of about 100 kJ a day (24 kcal) eventually takes off about 1 kg. But only half of that arrives in the first year, and about 95% by the end of the third. So the 7,700 rule is close for the first few weeks of a plan and grows more optimistic the longer the plan runs. Their online Body Weight Planner models the slowdown for your own numbers if you want the real curve.

The practical reading: keep the deficit by eating a little less again as you lose, rather than expecting one fixed intake to carry you to the target.