Calorie deficit: what it is and how to work it out

To lose weight, your body needs to use slightly more energy than it takes in: that gap is the calorie deficit. The principle is simple; putting it into practice is less so. Here is how to estimate it, how large it should be, and why a deficit that is too big often backfires.

What is a calorie deficit?

Every day your body uses energy: to keep running at rest (your basal metabolic rate), to digest, to move and to exercise. When your meals provide a little less than this total, the body draws the difference from its reserves, mainly body fat. That daily shortfall, counted in kilocalories, is the calorie deficit.

Without a deficit, weight stays the same or goes up, whatever the diet is called. Every approach that works creates a deficit in one way or another.

How to work out your calorie deficit

Estimate your total daily expenditure from your age, height, weight and activity. The easiest way is our calorie calculator, which uses the Mifflin-St Jeor equation and takes your exercise sessions into account.
Subtract a reasonable percentage from it, usually 15 to 25%.
Check that the result does not fall below a safety floor: 1,200 kcal a day for a woman and 1,500 kcal for a man, unless you are under medical supervision.

Example: someone who uses about 2,200 kcal a day would aim for about 1,750 kcal with a 20% deficit, that is 450 kcal less each day.

How big should the deficit be?

The bigger the deficit, the faster the early weight loss, but the harder it is to keep up. A deficit of 15 to 25% of your total expenditure, which usually means losing 0.5 to 1% of your body weight a week, is a good balance: progress is visible, hunger stays manageable and muscle is better preserved.

This is how the cure BalenZ programme works: a slightly larger deficit in the first month, when motivation is highest, a gentler one in the following months, then a stabilisation phase to keep the result.

3,500 kcal per pound: a rule to use with care

You often read that a kilo of fat is worth about 7,700 kcal (3,500 kcal per pound), so a 500 kcal daily deficit should take off just under half a kilo a week. It is a useful rule of thumb for the first weeks. Over time, real weight loss is slower than this rule predicts: as you get lighter, your body uses a little less energy and the deficit shrinks by itself (Hall et al., 2011). That is why our time estimates include a safety margin. See our weight-loss time calculator.

Mistakes that undermine a deficit

Going too low: a very low intake costs muscle, increases hunger and makes weight regain more likely afterwards.
Guessing portions: amounts judged by eye are often underestimated, especially oils, sauces, cheese and drinks.
Weekend catch-up: two days well above plan can wipe out a good part of the week’s deficit.
Forgetting protein: during a deficit, enough protein helps preserve muscle. See our protein calculator.
Judging too soon: weight goes up and down day to day with water and salt. The trend over three or four weeks is what matters.

Do you have to count calories for ever?

No. Counting helps you understand your needs and spot what weighs most in your diet. What lasts, though, is meals built with the right proportions from foods you enjoy. That is why the cure BalenZ programme turns your deficit directly into day-by-day menus, with no counting on your part: see How it works.

A calorie deficit is not suitable for everyone: it is not advised during pregnancy or breastfeeding, for teenagers, or with a history of eating disorders. If you have a chronic condition or take regular medication, ask your doctor first.

The cure BalenZ is a personalised nutritional rebalancing programme. It is neither a medical treatment nor medical advice, and does not replace a consultation with a health professional.