What a calorie deficit actually is

By My Calorie Pal · 29 October 2026

A calorie deficit means using more energy than you take in from food and drink. When that happens, your body makes up the difference from its own stores, and over time that's what makes you lose weight. Everything else in weight loss, from diets to fasting to cutting carbs, works by creating one.

Energy in, energy out

A calorie (strictly a kilocalorie, or kcal) is a unit of energy. Your body takes energy in through what you eat and drink, and uses energy to stay alive and move about. Your weight changes depending on the balance between the two:

• Calorie deficit: you use more than you eat, and you lose weight.
• Calorie surplus: you eat more than you use, and you gain weight.
• Maintenance: the two roughly match, and your weight stays about the same.

For example, if you burn 2,300 kcal in a day and eat 1,800, you're in a 500 kcal deficit for that day.

Where your calories go

Most people underestimate how much energy they use just existing. Your daily total, called your total daily energy expenditure (TDEE), is made up of three parts:

• Basal metabolic rate (BMR): the energy your body needs at rest to breathe, pump blood, keep warm and run your organs. For most people this is the biggest share, around 60 to 70%.
• Digesting food: breaking down and storing what you eat uses roughly 10% of your intake. Protein takes more energy to digest than fat or carbohydrate.
• Movement: both deliberate exercise and everything else, like walking, standing, fidgeting and housework. This part varies the most between people and from day to day.

You can estimate your own TDEE with the TDEE calculator, or use the calories burned figure from a fitness tracker.

Where the missing energy comes from

When you eat less than you use, your body still needs that energy, so it draws on what it has stored. Mostly that means body fat, along with some stored carbohydrate (glycogen) and, to a lesser degree, muscle.

A pound of body fat holds roughly 3,500 kcal, or about 7,700 kcal per kilogram. That's where the familiar rule comes from: a deficit of 500 kcal a day adds up to 3,500 a week, or about a pound of fat.

It's a useful rule of thumb rather than an exact law. Early on, a lot of the weight you lose is water stored alongside glycogen, so the scale often drops faster than the maths predicts. Later, as you get lighter, you burn a little less each day, so the same deficit produces slightly slower progress. Over months, it's a fair guide; over a few days, the scale can say almost anything.

Does it matter what you eat?

For how much weight you lose, the deficit is what counts. For how you feel while losing it, what you eat matters a lot. Foods high in protein and fibre keep you fuller for the same calories, which makes a deficit much easier to stick to. Enough protein also helps you keep muscle while you lose fat. So it isn't calories or food quality: you need the deficit to lose weight, and sensible food choices make the deficit bearable.

Common myths

"I barely eat and still can't lose weight"

This is almost always a counting problem, not a metabolism problem. In a well known study, people who said they couldn't lose weight on low calorie diets underreported what they ate by nearly half, and overreported their exercise by about half too. Nobody was lying; it's just very easy to miss oils, drinks, snacks and second helpings. Weighing food and reading labels closes most of the gap, which is what the meal calculator is for.

"Starvation mode stops weight loss"

Your body does adapt to eating less: you may move a little less without noticing, and your daily burn drops slightly as you get lighter. But this slows weight loss rather than stopping it. A genuine deficit always leads to weight loss over time.

"I can just exercise it off"

Exercise is good for you in almost every way, but it burns less than most people expect. A 30 minute run might use 300 kcal, which a large coffee and a pastry can replace in minutes. Exercise helps, but what you eat usually decides whether you're in a deficit.

A deficit doesn't have to happen every day

Your body doesn't reset at midnight. What matters is your balance over a longer stretch, and a week is a practical length to look at. A meal out or a big Saturday doesn't undo your progress if the rest of the week is in a deficit:

Saturday was well over, but the week as a whole was still a 1,300 kcal deficit. Looking only at Saturday, it feels like a failure; looking at the week, it's steady progress. That weekly view is exactly what the weekly tracker shows you.

The short version

• A calorie deficit means using more energy than you eat, and it's what causes weight loss.
• Most of the energy you use goes on simply staying alive; exercise is a smaller part than people think.
• About 3,500 kcal of deficit is roughly a pound of fat (7,700 kcal per kg), as a guide over weeks rather than days.
• Food quality decides how easy the deficit feels; the deficit decides the weight loss.
• Judge your progress by the week, not by any single day.

Next, you need a starting number: how many calories you burn in a typical day. Until the next chapter is ready, the TDEE calculator will give you a good estimate, and the calorie deficit calculator can turn a goal weight into a daily target.

Sources

1. NHS. Understanding calories.
2. Hall KD, Sacks G, Chandramohan D, et al. Quantification of the effect of energy imbalance on bodyweight. The Lancet, 2011; 378(9793): 826–837.
3. Lichtman SW, Pisarska K, Berman ER, et al. Discrepancy between self-reported and actual caloric intake and exercise in obese subjects. New England Journal of Medicine, 1992; 327(27): 1893–1898.
4. Wishnofsky M. Caloric equivalents of gained or lost weight. American Journal of Clinical Nutrition, 1958; 6(5): 542–546.

This guide is general information, not medical advice. If you have a health condition, are pregnant, or have a history of disordered eating, talk to your GP or a registered dietitian before changing how you eat.