How Many Calories Should I Be Eating?

Calories and energy are generally interchangeable words. Many are familiar with the oversimplified (and controversial) energy balance equation that states an energy balance can be achieved when energy intake equals energy output or expenditure. More simply, “calories in, calories out.”

Generally, an abundance of energy will lead to weight gain, while low energy availability or caloric deficit will lead to weight loss. To get a realistic idea of how many Calories you should be eating to meet your performance needs and health and weight goals, it’s important to understand how many Calories, or the amount of energy, you are expending daily. 

Why is Calorie capitalized? That is because the common reference to calories on nutrition labels actually refers to kilocalories, denoted by capital C. So, here’s the breakdown: 1 Calorie (with a capital C) = 1 kilocalorie (kcal) = 1,000 calories (small c). 

Science minute: In scientific terms, lowercase "calorie" denotes a unit of energy, defined as the amount of heat needed to raise the temperature of 1 milliliter of water 1° Celsius. Uppercase "Calorie", accordingly, is also a unit of energy, equal to 1,000 calories, or the amount of heat needed to raise the temperature of 1,000 milliliters of water 1° Celsius; these are also called kilogram calories (kcal). 

From most to least: Calculating RMR caloric needs

Total energy expenditure over a day is comprised of three components. 

The first and largest element of caloric expenditure comes from metabolism or resting energy expenditure. Basal metabolic rate (BMR) and resting metabolic rate (RMR) have slight differences but for estimations, and since BMR is more complex to measure, many of the validated caloric estimations are based on RMR – or the energy it takes to live: breathe, maintain body temperature, absorb nutrients, digest, conduct nerve signals, repair cells, sleep, and so on. For a sedentary person, RMR accounts for almost 70 percent of the day’s total energy usage, and a lesser percent the more active you are. Resting metabolism is influenced by age, sex, genetics, hormonal changes, body size and composition, exercise, environmental temperature, altitude, medications, and nutritional and lifestyle habits. 

Unless you have access to a metabolic cart for a direct measure of metabolic rate and know the exact values of your lean body mass, bone density, and fat mass, the first step to estimating energy needs is to calculate RMR from one of the many validated equations. 

Note that no equation is exactly perfect, and they may over- or under-estimate metabolism for certain populations. Despite the numerous limitations, it is still useful to approximate a starting number. So, break out your calculator and put on your math thinking cap.

The Mifflin-St. Jeor Equation¹ – This popular equation is a revised version of the early Harris-Benedict BMR equation from a century ago and is a good option if you know height, weight, age, and sex, although it does not consider ethnicity and has been found less accurate in obese people compared to non-obese.²

In kcals per day: 

Men: RMR = (9.99 x weight in kg) + (6.25 x height in cm) - (4.92 x age in years) + 6

Women: RMR = (9.99 x weight in kg) + (6.25 x height in cm) - (4.92 x age in years) - 161

In this case, a 45-year-old female who is 5 feet 6 inches (about 165 cm) and 145 pounds (65.9 kg) would have an approximate resting metabolic rate of 1,307 kcals (Calories) a day, before accounting for physical activity (we will get to that next). 

Cunningham Equation³ – This equation requires knowing the fat-free mass (FFM) of a person and, although it was validated on non-athletes, later research found it to be a valid predictor in athletic populations who tend to have higher FFM.⁴ 

RMR = 500 + (22 x FFM in kg)

If the same 45-year-old female (mentioned above) has 20-percent body fat, she has 52.7 kg of FFM. The Cunningham Equation estimates her RMR to be about 1,659 kcals a day, which is about 350 kcals a day more than the Mifflin-St. Jeor estimation. 

Simplified Resting Metabolic Rate Equation⁵ – Here is the easiest option and requires the least amount of math, derived from many of the equations used today. This estimation suggests men burn 1 kcal per kilogram of their current body weight each hour a day, and women burn about 10 percent less.⁶ 

Men: 1 kcal per kg of body weight x 24 

Women: 0.9 kcal per kg of body weight x 24

The RMR of our 45-year-old-female would be 59.3 kcals an hour and equal 1,423 kcals a day, which is about midway between the other two estimations.

Calculating physical activity caloric needs

The second component of energy expenditure includes physical activity (PA), which has the greatest ability to be controlled by the individual since it accounts for all movements during the day. 

Physical activity includes activities of daily living (ADL), like walking, standing, sitting, dressing, showering, or other personal care tasks; physical activity your job or profession may require like lifting, walking, or climbing stairs; and voluntary exercise like sports or gym exercises. For sedentary people, this might mean expenditure of only tens of Calories. But for those who do intentional exercise, it can account for hundreds (think a HIIT class or lifting session) or even thousands (a marathon or endurance triathlon) of Calories over a day. 

Short of having access to equipment that can provide a direct measure of oxygen consumption during physical activity, the more practical option is a wearable activity monitor. Another option is to use a metric that typically appears on cardio equipment at the gym. The Metabolic Equivalent (MET) is a value used to express a multiplication of energy expenditure above rest for an average metabolic reading rate. Charts also exist that have METS multipliers for hundreds of physical activities.⁷ 

If our 45-year-old female is walking on the treadmill for an hour and the METS are 3.3, then to calculate the energy burned during the walk it would be 59.3 kcals x 3.3, equaling 195.7 kcals. If she stopped after 30 minutes, then it would only be half, or if she increased the speed or incline and the METS went up, then the physical activity expenditure would, too. Of course, climate, elevation, fasted or fed state, and other factors can influence actual energy spent and, therefore, it is not a perfect equation.  

Calculating the thermic effect of foods

Third, the thermic effect of food (TE) typically has the smallest contribution, accounting for about 10 percent of total energy needs. It is, therefore, often not calculated nor added to RMR and PA values when estimating caloric needs because the effect is short-lived after a meal. TE includes the energy expenditure due to food consumption and varies based on meal type, amount, and frequency. For example, proteins have a greater effect than carbohydrates, and fats have the least. 

Therefore, the sum of the RMR daily needs and PA provides a decent estimate of energy expenditure for the day. 

The quick and easy daily energy needs calculation

Another option to consider is using the following chart that accounts for RMR and PA, used to ballpark estimations of daily energy needs, accounting for sex and activity level.⁵

Calculating daily caloric needs for weight gain or weight loss isn’t always as simple as manipulating energy consumed and energy expended, especially as weight and body composition change, or mobility or physical limitations change the ability to do ADL with age. The quality and types of foods and total calories, the amounts and proportions of macronutrients, the timing of meals, and hormones will also play a large role in determining a daily energy plan, maintaining satiation levels, and recovery after exercise and should be considered when assessing personalized energy needs with weight loss or weight gain goals.   

Ways to increase daily caloric needs

If you want to increase the number of calories your body burns on a daily basis, here are several tips to increase RMR, total energy expenditure, and PA values:

1. Exercise. The body burns calories during exercise, which would help increase the physical activity component of daily caloric needs, but it also burns energy after exercise while it is recovering from energy depletion. Higher intensity and longer duration exercise burns more calories and requires more replenishment, which translates to more overall energy expenditure.

Fuel properly for exercise with pre-workout and post-workout options.

2. Increase lean body mass. Weight in general will force the body to use more energy to survive, but the focus should be on maintaining or increasing fat-free mass or skeletal muscle mass, especially if you are trying to lose weight. Gradual weight loss, while maintaining lean body mass, will help preserve RMR levels.⁸

Thorne’s Creatine can promote the body’s production of lean body mass.*

3. Get restful sleep. Sleep is the time when your body repairs and recovers. Individuals who have restricted sleep might see declines in RMR because the body is seeking to conserve energy.⁹

Protein can be beneficial to sleep and muscle recovery. Add Thorne’s PharmaGABA to your protein in your evening routine for added benefits to your muscles while you sleep.*

4. Eat adequate calories. RMR is influenced by the number of calories consumed relative to energy expenditure. Excessive energy intake can increase RMR but also result in weight gain while fasting, and very low-calorie intake can support weight loss but also cause RMR to decrease.¹⁰ Low-energy availability can have numerous detrimental health effects. The key is finding the right amount for your goals – not too many, and not too few.

5. Reduce chronic stress. Stress can burn energy to help the body survive and maintain as close to homeostasis as it can – but whether you lose or gain weight depends on the duration of stress. Acute stress (like physical activity, the excitement of a needed vacation, or giving a presentation at work) can suppress appetite and reduce body weight. Chronic stress, however, can lead to over-eating, gaining fat mass, and weight and body composition changes,¹¹ all adversely impacting the efficiency of calorie expenditure. Recognize what is causing the bad stress in your life – test your cortisol levels, and get on a plan to reduce chronic stressors. 

References

1. Mifflin MD, St Jeor ST, Hill LA, et al. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr 1990;51(2):241-247.
2. Frankenfield DC. Bias and accuracy of resting metabolic rate equations in non-obese and obese adults. Clin Nutr 2013;32(6):976-982.
3. Cunningham JJ. A reanalysis of the factors influencing basal metabolic rate in normal adults. Am J Clin Nutr 1980;33(11):2372-2374.
4. ten Haaf T, Weijs PJM. Resting energy expenditure prediction in recreational athletes of 18-35 years: confirmation of Cunningham equation and an improved weight-based alternative. PLoS One 2014;9(9):e108460.
5. Dunford M, Andrew Doyle J. Nutrition for Sport and Exercise. Cengage Learning; 2014.
6. Ferraro R, Lillioja S, Fontvieille AM, et al. Lower sedentary metabolic rate in women compared with men. J Clin Invest 1992;90(3):780-784.
7. Herrmann SD, Willis EA, Ainsworth BE, et al. 2024 Adult Compendium of Physical Activities: A third update of the energy costs of human activities. J Sport Health Sci 2024;13(1):6-12.
8. Ashtary-Larky D, Bagheri R, Abbasnezhad A, et al. Effects of gradual weight loss v. rapid weight loss on body composition and RMR: a systematic review and meta-analysis. Br J Nutr 2020;124(11):1121-1132.
9. Spaeth AM, Dinges DF, Goel N. Resting metabolic rate varies by race and by sleep duration. Obesity 2015;23(12):2349-2356.
10. Molé PA. Impact of energy intake and exercise on resting metabolic rate. Sports Med 1990;10(2):72-87.
11. Rabasa C, Dickson SL. Impact of stress on metabolism and energy balance. Curr Opin Behav Sci 2016;9:71-77.