What Is a Calorie Deficit? How the Maths Works

A deficit is a difference, not a diet. Your TDEE is the energy you use in a day. If you eat less than that, the shortfall is the deficit, and the body makes up the difference from its stores. Every calorie target for losing weight is just TDEE minus a chosen gap.

The gap is the only thing the word describes. It says nothing about which foods to eat, when to eat them or how the target is split into protein, carbohydrate and fat. Those are separate choices.

From a daily gap to a weekly estimate

Calculators turn a daily deficit into an expected change in weight with a rule of thumb: a kilogram of body weight is treated as 7,700 kcal, which is the same as about 3,500 kcal per pound. The idea goes back to Wishnofsky's 1958 paper on the caloric equivalents of gained or lost weight. The estimate is the daily gap multiplied by seven days, divided by 7,700. Our calorie deficit calculator and the weight loss tools use exactly this step.

Working backwards gives a feel for the scale. A steady 0.5 kg a week needs a gap of about 550 kcal each day on this rule.

Worked example

Start from the man on the TDEE page, whose estimate is 2,759 kcal a day. The calculator offers fixed gaps, and each gives a daily target and a weekly estimate:

The middle row is the usual choice: a target of 2,259 kcal and about 0.45 kg a week. The weekly figure is the daily gap times seven, divided by 7,700.

Why it is only a rule of thumb

A body that weighs less uses less energy, so the same gap produces a smaller change as weeks go by. Hall and colleagues modelled this in 2011 and showed that the 3,500 kcal per pound rule overstates long-run change. For that reason every timeline on our calculators is the earliest sensible estimate, not a forecast. Your own weight record, kept over several weeks, is a better guide than any formula.

Limits we build in

The calculators treat 1,200 kcal a day for women and 1,500 kcal for men as a floor: the deficit calculator flags a target below it, and the weight loss tools stop there. The largest daily gap offered anywhere is 1,000 kcal. These are limits of the tool, set so it does not output an extreme number, and they are not individual advice. The CDC describes gradual, steady loss as about 1 to 2 pounds (0.5 to 0.9 kg) a week, which is the pace our timelines are compared against.

What a deficit does not tell you

A bigger gap is not automatically better, and a deficit that suits one person may not suit another. People who are pregnant, under 18, or managing a health condition should plan changes with a doctor or registered dietitian. The numbers here are a calculation, not a recommendation for you.

Logging what you eat is the practical way to see whether you are actually in a deficit, because the estimate only holds if the target is what you really eat. The opposite situation, eating above TDEE, is described under calorie surplus.