Calorie deficit calculator

A calorie deficit means eating fewer calories than your body uses over a period of time. Your body covers the shortfall from its own stores, and you lose weight. There is no way around this part: every diet that works, works because it creates a deficit, whether it counts calories, cuts carbs, or shortens your eating window.

The usual rule of thumb is that roughly 7,700 kcal is equivalent to a kilogram of body mass (about 3,500 kcal per pound). That is the conversion this calculator uses to turn a weekly pace into a daily calorie number. It is a planning estimate, not a law of physics — real-world loss is bumpier than the arithmetic suggests, because water, glycogen, and digestive contents move around from day to day.

A deficit of roughly 15–25% below maintenance suits most people. In practice that is often somewhere between 300 and 700 kcal a day, which works out at roughly 0.25–0.7 kg (0.5–1.5 lb) a week. Faster is not better: the larger the deficit, the more of the loss tends to come from muscle rather than fat, and the harder the plan is to stick to once hunger and fatigue build up.

A useful test is whether you could hold the plan for a month without your training, sleep, and mood falling apart. If not, the pace is too aggressive — and a slower plan you actually finish beats a fast one you abandon in week three.

If you have a lot of weight to lose, you can generally sustain a larger absolute deficit than someone close to their goal, because the deficit is a smaller share of a larger total expenditure. As you get leaner, the sensible move is to shrink the deficit, not hold it.

When the pace you pick lands your target below about 1,200 kcal a day for women or 1,500 kcal for men, the calculator flags it rather than silently raising the number. Those floors are a widely used practical minimum: below them it becomes genuinely difficult to hit your protein, vitamin, and mineral needs from food, and the risk of losing muscle rises.

We show the honest number and the warning together so the timeline you see still matches the pace you chose. The fix is to pick a gentler pace — or, if there is a medical reason to go lower, to do it under supervision from a doctor or registered dietitian.

Take a 30-year-old man, 180 cm and 80 kg, with a desk job and a couple of gym sessions a week. Mifflin-St Jeor puts his BMR at about 1,780 kcal. At a lightly active multiplier of 1.375, his maintenance sits near 2,450 kcal a day.

He wants to lose 5 kg at 0.5 kg a week. That pace needs a deficit of about 550 kcal a day, giving a target near 1,900 kcal — comfortably above the safe floor — and a timeline of about ten weeks. If he doubled the pace to 1 kg a week, the target would fall to about 1,350 kcal: still above the male floor, but an aggressive plan that would be hard to hold for ten weeks and more likely to cost him muscle.

The estimate uses the Mifflin-St Jeor equation for resting metabolic rate and a standard activity multiplier for everything you do on top of that. Both are population averages: two people with identical stats can differ by a few hundred calories a day in reality.

Treat the number as a starting point, not a verdict. Track for two or three weeks, watch what your weight actually does, and adjust by 100–200 kcal if the trend is not moving the way the plan predicted. That feedback loop is what makes the number accurate for you specifically — and it is the reason logging matters more than getting the initial estimate perfect.