Calorie Deficit Calculator

A calorie deficit is when you’re eating fewer calories than your body is burning. If you’re getting fewer calories than your body needs to carry out its daily tasks, you’ll start burning fat and other energy stores, which helps you to lose weight.

But if you lose weight too quickly, this can be unsustainable and even dangerous for your body, so it’s important not to overly limit your calorie intake.

A calorie deficit calculator can help you to work out how many calories less you should eat in a way that’s safe and sustainable.

In this article, we’ll talk about the specifics of what a calorie deficit is, how to work out how much of a calorie deficit to aim for, and the best ways to achieve this deficit.

What is a calorie deficit?

A calorie deficit happens when you’re burning more calories than you’re consuming. It’s a tried and tested weight loss method that’s simple to follow and can lead to impressive results when done safely and correctly.

Your body needs energy in the form of calories to stay alive. Energy is needed for all the body’s basic functions, like breathing, growing and repairing your tissues, and pumping blood around the body.

The number of calories your body needs a day just to carry out these basic tasks is known as your basal metabolic rate. This is how many calories the body needs before you consider extra tasks like moving and exercising, which can burn extra calories.

A calorie deficit can be created by increasing how much energy the body burns, or by decreasing how much energy comes into the body in the form of calories.

Why a calorie deficit works for weight loss

If you are eating more calories than your body needs, the excess energy is stored as fat and carbohydrates for you to use later. This can lead to weight gain.

A calorie deficit means your body can’t get all the energy it needs from the food you’re eating, so it needs to turn to these energy stores. You’ll then start using your fat and carbohydrate stores to power your body, which leads to weight loss.

Eating too few calories can send your body into starvation mode, which can actually stop you from losing weight. This means it’s important to get the balance right between eating fewer calories and still getting enough calories to fuel your body.

What calorie deficit is good for weight loss?

When you’re trying to work out what your individual calorie deficit should be, a general rule is that it should be around 600kcal fewer than your recommended daily allowance.

The recommended daily allowance is 2500kcal for men and 2000kcal for women, so a non-specific deficit would be 1900kcal for men and 1400kcal a day for women.

However, lots of factors affect how many calories your body burns each day and can affect what your deficit should be, such as:

  • age
  • sex
  • body size and composition
  • health
  • activity levels

A moderate calorie deficit is good to aim for, but a significant deficit can be unhelpful. Low and very low energy diets are not sustainable in the long term and can actually counteract weight loss efforts in some cases.

The National Institute of Health and Care Excellence (NICE) recommends that low-energy diets (800 to 1200 kilocalories a day) and very low-energy diets (fewer than 800 kilocalories a day) should not be used for more than 12 weeks. They’re a tool for achieving a specific goal that’s used in specific circumstances rather than a long-term weight management solution.

This means it’s better to use a calorie deficit calculator tool to accurately calculate how many calories you should aim to eat a day.

How does the calorie deficit calculator work?

The calorie deficit calculator determines the number of calories you need to eat daily to achieve your weight loss goals by estimating your daily calorie needs, also known as Total Daily Energy Expenditure (TDEE) and then taking away a calorie deficit amount based on your desired rate of weight loss.

There are 2 important factors to consider when calculating a calorie deficit. They are:

  1. Basal metabolic rate (BMR) – the minimum number of calories needed to maintain the body’s basic functions, such as breathing, digestion, and circulation.
  2. Total daily energy expenditure (TDEE) – the number of calories you burn in a day based on factors, like your age and activity level.

The calculator estimates your BMR based on your age, gender, weight, and height. It then estimates your TDEE by applying an activity level multiplier and subtracts your desired calorie deficit from your TDEE. This will provide you with a recommended daily calorie intake to help you reach your desired weight loss.

Tips for achieving a calorie deficit

You can achieve a calorie deficit by:

  • reducing the number of calories you’re eating
  • increasing your activity levels
  • a combination of both

Some people find it easier to cut back on the calories they’re eating to achieve a calorie deficit. This is because it can take a lot of extra exercise to burn a relatively small number of calories. For example, walking for 45 minutes burns an average 216 calories.

But it is important to exercise regularly as part of losing weight. Exercise that focuses on building muscle, like resistance and strength training, is great as it can help to increase your basal metabolic rate, allowing you to burn more calories at rest.

For most people, achieving a calorie deficit means calculating how many fewer calories they need to eat a day and planning how to do this.

It can help to write down what you would normally eat in a week or average day, and calculate how many calories you usually eat. That way, you’ll know where you’re starting from.

Breaking down your calories across the meals you’re going to eat is helpful for visualising your target. For example, a woman who is aiming to eat 1400 calories a day to achieve a deficit could eat:

  • 280kcal for breakfast
  • 420kcal for lunch
  • 420kcal for dinner
  • 280kcal as snacks and drinks

At least while you’re getting started, it can be useful to track the calories you’re eating in each meal, either using an app or by writing it down. You should also include the calories in any snacks, drinks, sauces, toppings, and oil that you cook with, as these calories can easily add up.

Although you’re aiming to eat fewer calories, it’s still essential for your body to get all the nutrients and vitamins it needs. This means eating a varied diet with at least 5 portions of fruit and vegetables a day and a variety of lean protein sources like beans, lentils, chicken, and fish.

With time, a calorie deficit can help you lose weight, but results are not instant. It’s ok if you ate too many calories in a meal or day – rather than seeing it as a reason to give up, just start again at the next meal.

FAQs

Can I build muscle while in a calorie deficit?

Yes, you can absolutely build muscle while in a calorie deficit, although it may be a little more challenging than when you’re eating normally.

You’ll need to make sure you’re eating plenty of protein and including a lot of resistance training in your workout routines. Make sure to also have rest days in between workouts to give your muscles time to rest and recover.

Why am I not losing weight even with a calorie deficit?

There are a few reasons why you might not be losing weight even with a calorie deficit.

First, it’s always good to check that you’re being accurate with tracking your calories. Make sure you’re including snacks and all the added extras, but also that you’re being realistic about the amount of calories in the portions you’re having – weighing your portions can help with this.

Second, your body may be adapting to the lower amount of calories you’re eating. Make sure to keep going with your reduced-calorie diet and increased amount of exercise, but check that you’re not overly restricting your calories to less than 800 a day.

If you’re still not losing weight, there may be an underlying medical reason, so it’s best to make an appointment with your GP and get things checked out.

How can I maintain energy levels in a calorie deficit?

When you start eating fewer calories than normal, you might find your energy levels drop. With time, your body should get used to running on fewer calories, and your energy levels should improve.

In the meantime, make sure you’re getting enough sleep, exercising regularly, drinking enough water, and eating a balanced diet that includes all the nutrients you need.

Do I have to be in a calorie deficit while being on Mounjaro or Wegovy?

Yes, for the best results when using weight loss injections like Mounjaro and Wegovy, you should maintain a calorie deficit.

These drugs can make you feel fuller for longer and reduce your hunger levels, but they won’t cause you to lose weight by themselves. You’ll still need to increase your exercise and eat fewer calories in order to lose weight.

Dr Zoe Miller

Medically reviewed by:

Dr Zoe Miller

Last reviewed: August 24, 2025

Medical Editor

Zoe initially studied biological sciences at Aston University, with an integrated placement year working as a mental health care assistant and volunteering abroad. After completing her first degree, she went on to study graduate-entry medicine at Warwick University. After graduating as a doctor, she worked within the West Midlands in Urology, Respiratory Medicine, Infectious Disease, and Psychiatry before transitioning into a full-time medical communications role. Zoe currently works as a freelance medical writer and editor for various health communications agencies, charities, pharmaceutical companies, and health organisations.

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