Physical activity level is a number in a formula that nutritionists, fitness professionals, and doctors use to determine how many calories per day a person burns, and therefore how many calories he or she should be eating. Scientists have used estimates of the energy expended doing various activities and sorted them into categories based on intensity and frequency, ranging from extremely inactive to extremely active. Due to the fact that all three of the components of the formula are based on estimates, there is some room for error. For most people, however, it is a fairly effective way to determine the amount of energy expended every day.
How much energy a person expends in a 24-hour period that is accounted for by physical activities, such as cleaning, working, and exercising, is a person's estimated physical activity level. This can then be turned into a multiplier, which is multiplied by the basal metabolic rate (BMR) to determine an estimate of a person's total energy expenditure (TEE) for a single day. If the basal metabolic rate and total energy expenditure are already known, you can find the physical activity level multiplier by dividing the total energy expenditure by the basal metabolic rate.
Basal metabolic rate is the amount of calories the body burns completely at rest, with no digestion, movement, or any other factors involved. These calories are used to supply all of the body's organ systems with enough energy to function and keep the body alive. If you already know your total energy expenditure, you can divide it by your physical activity level to determine your basal metabolic rate.
A person's physical activity level could be one of five or six categories, depending on the source used, each with its own multiplier assigned to it that estimates the amount of calories burned performing the tasks associated with it. A completely inactive person, such as someone who is bedridden, has a multiplier of 1.2. At the opposite end of the spectrum, an extremely active person, such as someone who has both a physically demanding job and participates in physically intense hobbies and sports, can have a multiplier of 2.0 or even 2.4.
The total energy expenditure, the result you get from multiplying basal metabolic rate by physical activity level, can help you gain, lose, or maintain weight. If, for example, your total energy expenditure is estimated to be 2,000 calories, you could make sure to eat an extra 500 calories, for a total of 2,500 per day, if your goal was to put on weight. Likewise, you could subtract 500 calories and eat 1,500 if your goal was to lose weight.