How Many Calories Do You Really Need? Calorimetry Can Tell You!

using metabolic equations to predict your needs

By Michael Ormsbee, PhDFlorida State University
Edited by Kate Findley and proofread by Angela Shoemaker, Wondrium Daily

You can scientifically measure energy expenditure, or how many calories you burn each day, using methods such as calorimetry. Professor Ormsbee explains.

Variety of fruits and vegetables on table with calculator and sticky note
A basal rate of everyday activities while at rest, plus eating and exercising, all fit into the metabolic calculations needed to determine a person’s nutrition and exercise plans for maintaining energy balance. Photo By conejota / Shutterstock

Indirect Calorimetry Calculations

By applying calorimetry to understand how much of what type of fuel you are burning for energy, you have a good idea of energy expenditure, which you need along with your calorie intake in order to calculate your overall energy balance and primary nutrient needs for your lifestyle. Even better, you will understand how the foods you eat impact this balance and ultimately, why and how eating and exercising contribute to your body composition.

Knowing that the average woman weighs 156 pounds (lbs), or 71 kilograms (kg), we can calculate her energy expenditure using indirect calorimetry. At rest, about 3.5 milliliters (ml) of oxygen are consumed per minute.

Thus, to learn how much oxygen she uses every hour, multiply 3.5 ml times 60 minutes. She uses 210 ml of oxygen per hour. Then we multiply her oxygen use by her weight in kg, which is 71 kg. 

Then we calculate: 210 ml times 71 kg equals about 15,000 ml or 15 liters of oxygen per hour. Multiplying this out over 24 hours, our subject consumes 360 liters of oxygen per day. Knowing this, we can multiply her daily oxygen intake—360 l—by 4.86 calories, the number of calories burned from a typical mixed diet of carbohydrates, proteins, and fats. We come up with an indirect calorimetric energy expenditure of roughly 1,750 calories burned per day just at rest.

Now, these 1,750 calories don’t include the energy this woman would need for activities like digesting food, walking around, and exercising. Thus, you can see that she would want to eat above 1,750 calories to account for these other daily activities to stay within energy balance. 

Most universities will offer this type of service if you’re interested in an evaluation for yourself. They can give you some detailed numbers to understand exactly what your resting metabolic rate is.

Estimating Energy Expenditure

Luckily for us, many predictive equations have been developed to estimate energy expenditure. These equations are based upon factors such as age, sex, height, weight, and daily activity levels.

Some commonly used metabolic equations include the Harris-Benedict equation and the Cunningham equation. The Harris-Benedict equation differentiates between males and females and uses height, weight, and age to evaluate metabolic rate. 

“What I like about the Cunningham equation is that it also takes into account your lean body mass,” Professor Ormsbee said.

These are just two of many predictive equation possibilities. When these equations are used, the value given indicates the person’s resting metabolic rate—when he or she is not doing any exercise, for example. The resting value then needs to be multiplied by an activity factor to get the actual daily needs that account for exercise and activity patterns that you normally take part in.

Activity Factors Range

Activity factors range from about 1.2 to 2.0. For example, you would multiply by about 1.3 for sedentary people, or those just doing light activity. 

To estimate actual calorie needs, you would multiply by about 1.7 for people who exercise daily at a moderate to high intensity; you would multiply by 2.0 or more for high-intensity exercise training and for people who work out more than once per day. Thus, the coefficient is larger for those who do more exercise, which makes sense. 

It is important to remember that these equations are only estimations. Metabolism is constantly changing based upon the needs of the individual, so it is important to re-evaluate a person’s nutrition plan based on performance, body weight, and body composition over time.

This article was edited by Kate Findley, Writer for Wondrium Daily, and proofread by Angela Shoemaker, Proofreader and Copy Editor for Wondrium Daily.
Dr. Ormsbee is an Associate Professor in the Department of Nutrition, Food, and Exercise Sciences and Interim Director of the Institute of Sports Sciences and Medicine in the College of Human Sciences at Florida State University.

Michael Ormsbee is an Associate Professor in the Department of Nutrition, Food, and Exercise Sciences and Interim Director of the Institute of Sports Sciences and Medicine in the College of Human Sciences at Florida State University. He received his MS in Exercise Physiology from South Dakota State University and his PhD in Bioenergetics from East Carolina University.