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How Many Calories Do You Really Need in a Day?

Cut through the calorie confusion with this guide to the basics
12th Apr 2022

What is a calorie? 

The precise scientific term kilocalorie (or kcal) is technically defined as the amount of energy needed to increase the temperature of one litre of water by 1°C, at sea level. This is the familiar figure displayed on the nutritional information of packaged foods, and is used to calculate energy intake. While one kilocalorie is actually equal to 1,000 calories in scientific terms, the word ‘calorie’ is more usually used to refer to kilocalories when we talk about nutrition. In this article when we use the term ‘calories’ it refers to kcals. 

So how many calories do we actually need? 

The honest answer to this question — although admittedly not the most satisfying — is that it depends on the person. Not everyone has the same energy requirements, as these will vary depending on an individual’s age, gender, weight, height, activity level and health status (1). 

Caloric needs are calculated based on your total energy expenditure (TEE) which refers to the amount of calories/energy burned on a given day. TEE is made up of a combination of resting energy expenditure, activity energy expenditure and the thermic effect of food. 

The majority of calories burned in a day are from your resting energy expenditure (REE) and used in homeostasis, or the maintenance of essential bodily functions such as breathing, blood circulation, thermoregulation, moving components in and out of cells and other physiological processes. Activity energy expenditure (AEE) refers to how much activity you do in a day and at what intensity level, e.g. whether you’re going to the gym or walking every day, or living a more sedentary lifestyle. The final component in making up our energy expenditure refers to the thermic effect of food (TEF). This is the energy your body uses to digest and absorb the food you have consumed (2). 

Taking all of this information into consideration, government guidelines in Ireland suggest that the average woman requires approximately 1,800-2,000 kcals a day depending on activity level, with more sedentary people fitting into the lower end of this bracket and more active individuals at the higher end. The same concept applies to men, but the suggested range for them lies between 2,000 and 2,500 kcals a day (3). It is important to understand that these figures are not set in stone, but are merely averages to use as general public health guidelines. Someone like a professional athlete may require far more calories than this general recommendation, while an individual who is less active may require fewer. 

There are many energy equations available to calculate a more individual caloric requirement, such as Mifflin St. Joer (1990), which has been shown to be a reliable predictive equation (4). Equations like this take into account body weight, height, age and gender to calculate your RMR. Additional activity-related energy requirements can then be added on via physical activity levels (PAL); this can be easily accessed in the Nutritics app, if you need more information on energy requirement calculations — we have a more detailed post here. If you are unsure of your current activity status, it always helps to keep an activity journal for a week and record your normal routine. Libro offers a convenient way to track diet and activity on your phone. 

Where do calories come from? 

The bulk of calories come from the three main macronutrients: carbohydrates, proteins and fats. One gram of carbohydrate or protein contains approximately four kcals, while one gram of fat has approximately nine. Once you know the gram weight of macronutrients in a food, you can calculate how many calories — or how much energy — that food contains. 

In this example, a 40g chocolate bar has 215 kcals. These are made up of: 
22.9g of carbohydrates (22.9 x 4 = 91.6 kcals) 
2.9g of protein (2.9 x 4 = 11.6 kcals) 
12.4g of fat (12.4 x 9 = 111.6 kcals) 
Total calories (91.6 + 11.6 + 111.6 = 214.8 kcals), which rounds to 215 kcals. 

Knowing which macronutrients make up your food can potentially lead to better food choices. 

Why do I need to know how many calories I need? 

Knowing how many calories you need in a day enables you to maintain a healthy weight and to support bodily functions, balancing the number of calories you consume through food and drink with those burned through physical activity. Think of a carefully balanced see-saw: if the see-saw leans one way, you lose weight, and if it tips the other, you gain weight. Your maintenance levels are also useful when choosing to lose or gain weight; eating 300-500 kcal above or below your maintenance level is recommended to achieve the desired weight goal (5). 

5 tips for building good habits at meal time

1. Know your food choices. Being aware of what is in food is very important in controlling your calorie intake. Foods containing high amounts of fat can lead to a high calorie count — something as small as one teaspoon of peanut butter has 85 kcals — so it may be wise to portion them out tactically or opt for low-fat dairy products and leaner meats. You should also be mindful of portion sizes when it comes to sauces, as they may contain fat and sugar. When consumed in liquid form, it’s easy for calories to go unnoticed. Drinking still/sparkling water instead of sugary fizzy drinks or other calorie-dense beverages will save a lot of extra calories at meal time. It’s also important to remember that alcohol contains calories too; in fact, every gram of alcohol contains 7 kcals, more than either carbohydrates or proteins, so be aware of how much you drink with your meal. Nutritics’ recipe function has a built in traffic light system to provide detailed information of the nutritional profile of a dish. 

Click the light bulb icon beside a nutrient to see suggestions provided for ways to reduce the amount of this nutrient in your recipe, e.g. as seen with fat in the image below.

2. Know your labels. Being able to read labels can also help you make smart food choices, based on their calorie content and nutritional profiles. A useful way of monitoring this is the ‘traffic light’ colour coding system. 
  • Green means this food is a healthy choice, low in fat, salt or sugar. If you buy a food that has all or mostly green on the label, you know straight away that it's a healthier choice. 
  • Amber indicates neither high nor low levels, so you can eat foods with all or mostly ambers on the label most of the time. 
  • Red on the label means the food is high in fat, salt and/or sugars. These are the foods that should be limited to occasional eating. 

The labels on packaged foods can also provide you with helpful information around ingredients, macronutrient breakdown and fibre content, and can even suggest a suitable serving size per person to avoid consuming too many calories. 

3. Consider your cooking methods. Choose to grill, steam and boil foods instead of frying them — oil contains a lot of fat and hence a lot of calories. If cooking with oil, try not to add more than one teaspoon per serving and use nonstick cookware. Not only does this eliminate the need for lots of oil, but it’s easier to clean, too.
4. Improve satiety. Include more whole grains at meal time to regulate appetite. The fibre content in these complex carbohydrates means they take longer to be digested and will keep you full for longer (6). This can also be said for protein, the most satiating macronutrient — meaning it takes the most amount of energy to metabolise fully, and helps you feel satisfied for longer after eating (7).
5. Know how to build a calorie-friendly meal. When building a calorie-friendly meal, National Healthy Eating Guidelines are designed to help you. Not only do they help guide your portion sizes, but it also acts as a checklist, reminding you to include those all-important vegetables and whole grains. Different countries provide different guidelines and visual representations, e.g. Ireland uses the Food Pyramid and the UK uses the EatWell Guide. 


  1. Osilla, E. V., Safadi, A. O., & Sharma, S. (2021). Calories. In StatPearls. StatPearls Publishing.
  2. Hall, K. D., Heymsfield, S. B., Kemnitz, J. W., Klein, S., Schoeller, D. A., & Speakman, J. R. (2012). Energy balance and its components: implications for body weight regulation. The American journal of clinical nutrition, 95(4), 989–994.
  3. Food Safety Authority of Ireland. (2011). Scientific recommendations for healthy eating guidelines in Ireland. Food Safety Authority of Ireland
  4. Frankenfield, D., Roth-Yousey, L., & Compher, C. (2005). Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. Journal of the American Dietetic Association, 105(5), 775–789.
  5. Raynor, H. A., & Champagne, C. M. (2016). Position of the Academy of Nutrition and Dietetics: Interventions for the Treatment of Overweight and Obesity in Adults. Journal of the Academy of Nutrition and Dietetics, 116(1), 129–147.
  6. Barber, T. M., Kabisch, S., Pfeiffer, A., & Weickert, M. O. (2020). The Health Benefits of Dietary Fibre. Nutrients, 12(10), 3209.
  7. Leidy, H. J., Clifton, P. M., Astrup, A., Wycherley, T. P., Westerterp-Plantenga, M. S., Luscombe-Marsh, N. D., Woods, S. C., & Mattes, R. D. (2015). The role of protein in weight loss and maintenance. The American journal of clinical nutrition, 101(6), 1320S–1329S.