What Is a Calorie Deficit and How Does It Work?

What is a calorie deficit?

A calorie deficit means your body uses more energy than you consume from food and drink over a period of time. The body then has to draw on stored energy to make up the difference.

A deficit is therefore about the relationship between energy intake and energy expenditure, not about one particular food or exercise.

Want to try the idea yourself? Use the Calorie Deficit Calculator.

How TDEE fits in

Total Daily Energy Expenditure, or TDEE, is an estimate of how many calories you use in a typical day. Eating below that estimated level creates an estimated deficit.

For example, if TDEE is around 2,400 calories and average intake is 2,000, the estimated daily deficit is 400 calories.

If you want to explore this topic further, see BMR vs TDEE: What Do They Mean?.

Why the calculation is only an estimate

Food labels, portion sizes, activity trackers and TDEE formulas all have margins of error. Your actual energy expenditure also changes from day to day.

It is therefore more useful to think in averages and trends than to assume a calculated deficit is exact to the calorie.

Is a bigger deficit better?

Not necessarily. Very aggressive restriction can be difficult to sustain and may make it harder to meet nutritional needs. Individual needs also differ depending on health, age, body size and activity.

A calculator should help you understand the maths, not encourage extreme targets.

Estimate the numbers

The Calorie Deficit Calculator can compare an estimated daily energy requirement with a calorie target so you can see the size of the difference.

If you are unsure what intake is appropriate for you, particularly if you have medical or nutritional concerns, seek personalised professional advice.