How big a calorie deficit do you actually need?

A calorie deficit is simply the gap between energy in and energy out. If you burn 2,500 kcal on a typical day and eat 2,000, you are in a 500 kcal deficit, and your body covers the shortfall from its own stores. Sustained over time, that is what weight loss is.

The number you are measuring the gap against is your maintenance level, also called your TDEE. Everything below depends on having a reasonable estimate of that figure first, because a deficit is a relative quantity: 2,000 kcal a day is a deep cut for one person and a surplus for another.

The standard rule of thumb treats stored body fat as roughly 3,500 kcal per pound, or about 7,700 kcal per kilogram. Divided across a week, that gives the two figures most calculators use:

FitQuota's calorie calculator uses a 250 kcal daily adjustment for its lose and gain goals, which lands at the top row: about half a pound a week. That is a deliberately modest default, for the reason in the next section.

The rule assumes two things that are not true. It assumes every pound you lose is pure fat, when some of it is lean tissue and water. And it assumes your energy expenditure stays fixed, when in fact a lighter body burns fewer calories doing exactly the same things.

Kevin Hall's 2008 analysis of this arithmetic makes the practical consequence clear: applied over months rather than weeks, the rule predicts substantially more weight loss than people actually achieve, because the deficit shrinks as the body does. A 500 kcal gap against your starting maintenance level is a smaller gap against your maintenance level twenty pounds later.

This is not a reason to distrust the number; it is a reason to treat it as a starting point that gets recalculated. Plateaus after a couple of months are usually this effect, not a failure of willpower or a “broken metabolism”.

Bigger deficits lose weight faster and are harder to sustain, and the weight you lose in a steep deficit contains more lean tissue. Most general guidance lands on 0.5–1 lb (roughly 0.25–0.5 kg) a week for this reason, which is a daily deficit somewhere between 250 and 500 kcal for most adults.

Two practical limits are worth stating plainly. A deficit that takes you far below your BMR is unlikely to be a good idea without supervision, because that is the energy your body needs before you have done anything at all. And very low intakes make it genuinely difficult to hit protein, fibre and micronutrient targets, which is a nutrition problem rather than a calorie one.

Longer than the arithmetic suggests, because day-to-day weight is mostly water, food in transit and glycogen. A 500 kcal daily deficit moves about 0.45 kg of actual tissue in a week, which is comfortably inside the range your weight swings anyway between a Tuesday and a Friday.

Two to three weeks of consistent intake is roughly the shortest window in which a trend becomes readable, which is why weighing on the same day, at the same time, and looking at the average rather than any single reading, tells you far more than the number this morning.