What Is a Calorie Deficit and How to Calculate Yours
A calorie deficit means your body uses more energy than you take in from food and drink. To cover the gap, the body draws on stored energy, mostly body fat. In Australia energy is measured in kilojoules (kJ): one calorie (kilocalorie) is 4.2 kJ.
This is general information, not medical advice. Rates of weight loss vary between individuals, and a GP or accredited practising dietitian can help you set a target that suits your health.
What Does Calorie Deficit Mean?
Your body needs energy to function and move, and that energy comes from food and drink. Take in more than you use and the extra is stored as fat. Use more than you take in and the body draws on its stores. When the two are about equal, weight stays about the same.
A calorie deficit is the size of that gap. A deficit of 2,500 kJ a day means your body uses 2,500 kJ more than you eat and drink across the day.
| Energy in | What tends to happen |
|---|---|
| More than you use | Extra energy is stored, mostly as body fat |
| About the same as you use | Weight stays about the same |
| Less than you use (a deficit) | The body draws on stored energy, mostly fat |
Kilojoules and Calories
Australian food labels list energy in kilojoules, while many apps and overseas websites use calories. To turn calories into kilojoules, multiply by 4.2. To turn kilojoules into calories, divide by 4.2. So a 2,500 kJ deficit is about 600 calories.
How to Calculate Your Calorie Deficit
Working out a deficit takes three numbers: roughly how much energy you use each day now, your daily target at a healthy weight, and the lowest intake that still makes sense.
Step 1: Estimate how much energy you use each day
The average adult needs about 8,700 kJ (about 2,070 calories) a day, but needs vary with how active you are, how much muscle you have, your stage of life, and your age, sex, height and weight (healthdirect).
For a personal estimate, the Australian Government's free daily energy requirements calculator on Eat for Health works it out from your details. Enter your current weight to see roughly what your body uses now.
This daily figure is what many apps call your maintenance calories: the intake at which weight stays about the same. Any calculator gives an estimate, not an exact figure. Treat the result as a starting point you will adjust.
Step 2: Set your target at a healthy weight
Run the calculator again with your ideal body weight, a weight in the healthy range. Its instructions say that if you are aiming to lose weight, this gives a lower estimated energy requirement and may help you start losing weight. That figure is your daily target, used as it is, with nothing further taken off.
Step 3: Work out the gap and check the floor
Your deficit is the difference between step 1 and step 2. Australia's clinical guidelines for managing overweight and obesity (NHMRC, 2013) design eating plans for adults above a healthy weight around an energy deficit of about 2,500 kJ a day. That figure describes the size of the gap. It is not a further cut from your target.
You can widen the gap by moving more as well as eating less. The same guidelines pair reduced intake with about 300 minutes of moderate-intensity activity, or 150 minutes of vigorous activity, or a combination, each week, plus support for changing habits.
Then check the target. The Better Health Channel (Victorian Government) says a good weight management service will advise against a daily intake below 5,000 kJ (1,200 calories). If your target falls below that, talk to a GP or dietitian first.
A worked example
Take a woman aged 31 to 50 with seated work, 1.6 m tall and weighing 79 kg. The Nutrient Reference Values (Eat for Health) give these estimates:
uses now, at 79 kg: about 8,700 kJ (about 2,070 calories) a day
target, at 56 kg (a BMI of 22): about 7,600 kJ (about 1,810 calories) a day
gap: about 1,100 kJ (about 260 calories) a day
That target sits above the 5,000 kJ floor, so it is a reasonable starting point to discuss with a health professional. The table runs the same steps for other adults aged 31 to 50 with seated work. Calorie figures are rounded.
| Example | Uses now | Target (gap) |
|---|---|---|
| Woman, 1.6 m, 71 kg | 8,300 kJ | 7,600 kJ (700) |
| Woman, 1.6 m, 79 kg | 8,700 kJ | 7,600 kJ (1,100) |
| Man, 1.8 m, 88 kg | 11,000 kJ | 9,900 kJ (1,100) |
In these examples the gap is smaller than the 2,500 kJ in the national guidelines. Each person's gap depends on how far their weight is from a healthy weight and how active they are. As weight comes down, what the body uses comes down too, so the gap narrows.
As for what to expect on the scale, Better Health describes a gradual loss of around 1 to 4 kg a month to begin with as what a good weight management service recommends. That is a range, not a prediction: the NHMRC guidelines state that rates of weight gain and loss vary widely between individuals. For the weekly and monthly detail, see how much weight you can realistically lose in a month.
Tracking Progress
A single weigh-in says little. Look at the trend over several weeks, and measure your waist as well as your weight, because the aim is to lose body fat, not just total body weight. To see progress as a share of your starting weight, use our guide on how to calculate your weight loss percentage.
Ways to Create a Calorie Deficit
A deficit works best when it is one you can keep up. These approaches come from Australian guidance:
swap energy-dense foods such as takeaway, processed food, sugary drinks and alcohol for vegetables, fruit, wholegrains, legumes, nuts and seeds, lean meat, fish and eggs, and reduced-fat dairy
build meals around the five food groups in the Australian Dietary Guidelines, and drink plenty of water
read the nutrition information panel: Australian labels list kilojoules per 100 g (or 100 mL), which makes products easy to compare
aim for 30 minutes or more of moderate to vigorous physical activity on most days, plus muscle-strengthening activities on 2 or more days a week (Australia's physical activity guidelines)
break up long periods of sitting, and add light movement through the day
get 7 to 9 hours of good quality sleep, which the same guidelines list alongside movement
For more practical habits, see our guide to everyday habits that support weight loss.
Why the Quality of Your Food Still Matters
A calorie deficit is about quantity, but a healthy diet is about quality too. A large salad can hold the same number of kilojoules as a small block of chocolate, yet the salad carries more nutrients and keeps you full for longer (healthdirect).
The Australian Dietary Guidelines recommend being physically active and choosing amounts of nutritious food and drink to meet your energy needs, and limiting foods and drinks with saturated fat, added salt, added sugars and alcohol. To split your target into protein, carbohydrate and fat, see our guide to macros for weight loss.
Risks of Cutting Back Too Hard
Very restrictive fad diets carry risks, including (healthdirect):
feeling hungry all the time, with cravings and a bigger appetite
headaches, tiredness, dehydration or constipation
not getting enough vitamins and minerals
losing weight in a short time and then regaining it soon after
Weight loss also sets off a response in the body. Levels of the hormones that regulate appetite shift in ways expected to reduce the feeling of fullness after eating and increase hunger, and this shift does not reverse for at least a year after the initial loss. For that reason the NHMRC guidelines stress regular support over the long term, not a short burst of dieting.
Who Should Get Advice Before Starting a Deficit
A calorie deficit is not the right approach for everyone. Talk to a GP or accredited practising dietitian first if you:
are pregnant or breastfeeding, when energy needs go up, not down
are under 18, as children and teenagers need energy to grow
have diabetes, heart disease or another ongoing health condition
have a history of disordered eating
When a Deficit Is Not Enough on Its Own
For some adults living with obesity, diet and physical activity changes may not bring enough weight loss for their health goals. Treatment for obesity can include a lower-energy diet, more physical activity, weight loss medication and bariatric surgery. Any medication is used together with reduced kilojoule intake and increased activity, not instead of them, and it has side effects and is not suitable for everyone.
Whether any treatment beyond diet and exercise is appropriate is a decision for a registered healthcare practitioner after an individual assessment, and outcomes vary between individuals. See whether diet and exercise alone are enough and what happens at a weight loss consultation.
Frequently Asked Questions
How do I calculate my calorie deficit?
Estimate what your body uses now with a tool such as the Eat for Health daily energy requirements calculator, using your current weight. Run it again with your ideal body weight: that figure is your daily target, with nothing further taken off. The difference between the two is your deficit. Check that the target is not below 5,000 kJ (1,200 calories).
Is 1200 calories a day a good deficit?
Not as a starting point for most adults without advice. 1,200 calories is about 5,000 kJ, the lowest daily intake a good weight management service should advise, according to the Better Health Channel. For an adult with average needs of 8,700 kJ, that would be a far bigger gap than the 2,500 kJ in Australian guidelines. Talk to a GP or dietitian first.
Will a calorie deficit burn belly fat?
In a deficit the body draws on stored energy, mostly fat, and results vary between individuals. Waist size is a good indicator of the hidden fat around the organs in the belly, so measure your waist as well as your weight. A waist of 94 cm or more for males, or 80 cm or more for females who are not pregnant, carries a higher risk of health problems.
Is a 1,000 calorie deficit too much?
For most adults, yes, without professional guidance. A 1,000 calorie deficit is about 4,200 kJ a day, well above the 2,500 kJ figure in Australian clinical guidelines. For an adult with average needs of 8,700 kJ, it would leave about 4,500 kJ a day, below the 5,000 kJ floor. A GP or dietitian should guide any deficit that large.
How much weight will I lose in a calorie deficit?
It varies between individuals, so no one can promise a number. As a guide, Victorian Government health advice describes a gradual loss of around 1 to 4 kg a month to begin with, and calls about half to 1 kg a week reasonable. Judge progress over several weeks, not a single weigh-in.
References
Eat for Health (NHMRC). Australian dietary guidelines 1 - 5.
Eat for Health (NHMRC). Daily energy requirements calculator.
Better Health Channel (Victorian Government). Weight management services.
Better Health Channel (Victorian Government). Weight loss: a healthy approach.
This article is general information only and does not replace individual medical advice. Please speak with a registered Australian healthcare practitioner about your own circumstances.