Strength training, often referred to as resistance exercise or weight lifting, is a powerful activity for building muscle and improving overall metabolic health. Determining the exact number of calories burned during a session is not as simple as tracking a steady-state run, since the metabolic demands are highly varied. Unlike continuous aerobic exercise, strength training involves fluctuating periods of high exertion followed by rest, which complicates a straightforward calculation. The total caloric expenditure from lifting weights is a combination of the energy used during the workout itself and a sustained “afterburn” effect that continues long after the final set.
Estimating Calories Burned During the Session
To estimate the energy expenditure during the actual lifting session, scientists often rely on a measure called the Metabolic Equivalent of Task, or MET. One MET represents the energy your body expends while sitting at rest, which is roughly equivalent to one calorie per kilogram of body weight per hour. Activities are assigned a MET value that indicates how many times more energy they require than resting. Light to moderate effort resistance training is typically assigned a MET value of around 3.0 to 3.5, while a vigorous, high-intensity session can reach 6.0 METs or higher.
The basic formula to approximate calories burned involves multiplying the activity’s MET value by your body weight in kilograms, and then multiplying that number by the duration of the activity in hours. For example, a person weighing 70 kilograms (about 154 pounds) performing a moderate-intensity weightlifting session (3.5 METs) for one hour would burn approximately 245 calories. This calculation estimates the energy used during the active lifting time but does not account for the subsequent recovery process.
Key Variables Influencing Expenditure
The assigned MET value for strength training is a broad average, and the true calorie expenditure is heavily modified by the specific structure of the workout. The primary factor is the load and intensity, as lifting weights closer to your maximum capacity requires more motor unit recruitment and places a greater strain on the central nervous system. This increased physiological demand elevates the energy cost compared to lifting lighter weights for high repetitions.
Exercise selection also plays a substantial role, favoring compound movements that engage multiple large muscle groups simultaneously. Exercises like squats, deadlifts, and bench presses recruit far more muscle mass than isolation movements such as bicep curls or triceps extensions, demanding a higher overall calorie expenditure. The management of rest periods is another determinant of the caloric rate. Shorter rest intervals between sets prevent the heart rate and metabolic rate from fully recovering, forcing the body to sustain a higher level of energy output. Individuals with greater overall muscle mass naturally burn more calories during the same workout, as supporting more muscle tissue requires increased energy.
Understanding the Post-Workout “Afterburn” Effect (EPOC)
The most unique and significant component of strength training’s caloric impact happens after the workout is complete, a phenomenon known as Excess Post-Exercise Oxygen Consumption, or EPOC. EPOC represents the increased rate of oxygen consumption the body uses to restore itself to a pre-exercise state of homeostasis. This recovery process requires a sustained expenditure of energy, meaning the body continues to burn calories at an elevated rate for an extended period.
Strength training is particularly effective at generating a pronounced EPOC effect because it disrupts the body’s internal environment to a greater degree than steady-state aerobic exercise. This disruption involves several energy-consuming processes:
- The repair of micro-tears in muscle tissue.
- The replenishment of adenosine triphosphate (ATP) and phosphocreatine stores.
- The clearance of metabolic byproducts like lactate from the blood.
The intensity and duration of the lifting session directly correlate with the magnitude and length of the EPOC effect, which can last anywhere from a few hours up to 48 hours. Studies suggest that EPOC can account for an additional 6% to 15% of the total calories burned during the session, making it a substantial contributor to the overall energy cost.
Tools for Tracking and Measurement
While the MET formula offers a theoretical estimate, many people rely on consumer-grade fitness trackers and smartwatches to measure their calorie burn. These devices primarily estimate energy expenditure based on heart rate and movement data collected by accelerometers. For strength training, however, these tools often struggle with accuracy because the heart rate response does not correlate as predictably with energy expenditure as it does during continuous activities like running.
Fitness trackers can both overestimate and underestimate the calories burned during a resistance workout, with errors sometimes exceeding 20% compared to laboratory measurements. This inaccuracy stems from the intermittent nature of lifting, where high effort is followed by periods of relative rest, and from the fact that strength training’s intense nature can cause a heart rate lag. For a more informed estimate, one can use online calculators that apply the MET formula, which requires inputting activity intensity, duration, and body weight. Ultimately, it is best to use the calorie data from any device as a measure of relative effort, comparing one day’s workout to the next, rather than treating the number as an absolute, precise figure.