What Is a Calorie Deficit? A Simple Guide

6 November 20266 min read

A wooden chopping board next to some green vegetables. — Photo by Towfiqu barbhuiya on Unsplash.

A calorie deficit means taking in fewer calories from food and drink than your body uses. The NHS explains that when you’re in a calorie deficit, your body relies on its fat stores for fuel. Over time, that’s how you lose weight.

In brief

Your body uses calories all day, for breathing, moving and digesting food.
If you take in less than you use, the gap comes from your body's stores.
The NHS suggests a deficit of around 600 calories a day. A smaller one works too, just more slowly.
The scale shows a deficit as a trend over a few weeks, not day to day.
As you lose weight, your body uses a little less, so eating the same amount gives a smaller deficit.

What does a calorie deficit mean?

Calories are units of energy in food and drink. The NHS says your body uses this energy all day long for:

basic functions, like breathing and keeping your heart beating
movement and exercise
digesting the food you eat

The total is often called your maintenance (sometimes called TDEE, short for total daily energy expenditure), the amount that keeps your weight steady. A calorie deficit is any gap below that. For example, if your body uses about 2,000 calories a day and you eat 1,600, you’re in a deficit of about 400 calories a day.

The opposite is a calorie surplus. If you regularly take in more than your body uses, the NHS says the extra energy is stored as fat over time.

How does a calorie deficit work?

Your body needs a certain amount of energy each day. If your food and drink cover less than that, the NHS says your body relies on its fat stores to make up the rest.

A common rule says a kilo of fat holds about 7,700 calories (about 3,500 per pound). By that rule:

Daily deficit (calories)	Roughly per week	Roughly per month
250	0.2 kg (½ lb)	1 kg (2 lb)
500	0.45 kg (1 lb)	2 kg (4 lb)
600	0.55 kg (1¼ lb)	2.3 kg (5 lb)

The rule is only a rough guide. A 2014 paper by Thomas and colleagues found it overestimates weight loss over time, because your energy needs fall as you lose weight. More on that in how many calories are in a pound of fat.

Early on, the scale often drops faster, because some of that early loss is water rather than fat. After that, it moves more slowly and bounces from day to day.

How big should a calorie deficit be?

The NHS suggests cutting around 600 calories a day, which it calls a safe and sustainable way to lose weight. A smaller one can feel easier to keep up. Our calorie deficit calculator keeps to gentler paces, up to about 500 a day, and gives you a daily target. We also show example targets at different weights.

A bigger deficit isn’t always better. If you’re constantly hungry or tired, your deficit may be too big. Eating a little more is fine. Our calculator never gives a target of 1,200 calories a day or less, and here’s why.

Talk to a GP or a healthcare professional before changing your diet if you have a medical condition or a special diet for your health. The same goes if you have, or have had, an eating disorder.

How do you create a calorie deficit?

You can eat less, move more, or both.

To eat a little less, the NHS suggests splitting the cut across the day. For example:

Breakfast, about 100 less. Weigh your cereal, and skip jam or chocolate spreads.
Lunch, about 200 less. Check labels on shop-bought food, and choose water or sugar-free drinks.
Dinner, about 200 less. Have fewer ready meals and takeaways, and cook from scratch where you can.
Snacks, about 100 less. Swap crisps and chocolate for fruit or plain popcorn.

Moving more helps too, and everyday movement like walking counts as well as exercise. We explain how it adds up in what is NEAT. If you exercise to create a deficit, check whether you should eat back your exercise calories.

You don’t have to count calories to be in a deficit. But counting calories for a few weeks shows you where your calories come from.

How do you know if you’re in a calorie deficit?

Your weight trend tells you. Weigh yourself a few times a week at the same time of day, then compare weekly averages. If your average keeps falling over three or four weeks, you’re very likely in a deficit. If it’s flat for a few weeks, you’re probably eating close to your maintenance.

Feeling hungry doesn’t prove you’re in a deficit. Hunger depends on lots of things, like what you’ve eaten, how you slept and how active you’ve been. A single weigh-in doesn’t prove it either, because your weight changes every day. It can take a few weeks for a calorie deficit to show on the scale.

Why does weight loss slow down in a calorie deficit?

As you lose weight, your body uses a little less energy, so the same amount of food slowly becomes your new maintenance. It’s also easy to start eating a little more without noticing, or to underestimate portions.

If your weight has stopped falling for a few weeks, re-run the calculator with your current weight, and check you’re counting everything. There’s more to check in weight loss plateaus. It’s also why your calorie target shouldn’t stay the same.

Where Ori fits

Ori gives you a starting calorie target based on your body. With Premium, Ori checks your weight trend each week. If your weight isn’t moving as expected, it adjusts your target. Our Stealth Health mode, which is free, hides calorie and weight numbers and tells you how things are going in plain words. You can try Ori here.

Sources

A gentle safety note

Ori supports general wellness. It is not medical care. If you have a history of disordered eating, are pregnant or breastfeeding, take medicines that affect weight or appetite, or have a clinical nutrition need, speak with a qualified healthcare professional before changing your intake or weight goal.

Read Ori's Health Disclaimer