The Standard Activity Factors, also known as Harris-Benedict Standard Activity Factors (SAFs), are a simple way to classify a person’s activity level. The SAF scale ranges from sedentary to moderately active, all the way up to extra active.

Although associated with the Harris-Benedict equation, the SAF scale was first defined in the 1996 book Exercise Physiology by McArdle et al.

There are 5 Standard Activity Factors, with each representing a greater level of physical activity. Each level corresponds to a multiplier from x1.2 to x1.9.

Online calorie calculators use the SAF multipliers to adjust your basal metabolic rate (BMR). BMR is the number of calories your body needs at total rest to power your most basic functions. This includes the baseline activity of your organs, but does not include the digestion of food or the calories burned from physical movement and exercise.

Multiplying your BMR by your SAF gives your total daily energy expenditure (TDEE). TDEE is your maintenance calories – if you ate exactly at your TDEE, you would neither lose or gain weight.

Standard Activity Factor (SAF) Defintions

The Harris-Benedict Standard Activity Factor (SAF) scores range from 1.2 (+20%) to 1.9 (+90%), depending on your level of physical activity.

There are 5 SAF scores, each with a description of a person’s typical activity level. You will need to determine which SAF score fits your lifestyle best:

• Sedentary (SAF = 1.2) – Your daily life involves little to no exercise. You may work a desk-based job and spend your spare time indoors with little physical activity.
• Light Activity (SAF = 1.375) – You take part in light exercise or sports 1-3 days per week. If you work in a job where you spend time on your feet for most of the day, this may be an appropriate factor.
• Moderate Activity (SAF = 1.55) – You take part in moderate exercise or sports 3-5 days per week. This can include activities like jogging, cycling or swimming for at least 30 minutes each day.
• Very Active (SAF = 1.725) – You take part in moderate-to-vigorous exercise or sports 6-7 days per week. This can include running or playing competitive sports.
• Extra Active (SAF = 1.9) – You take part in vigorous training two times a day or have job requiring hard physical labor.

References

Frankenfield, D. C., Muth, E. R., & Rowe, W. A. (1998). The Harris-Benedict studies of human basal metabolism: history and limitations. Journal of the American Dietetic Association, 98(4), 439-445.

McArdle, W., Katch, F., & Katch, V. (1996). Exercise Physiology 4th edition Baltimore: Williams and Watkins.