The number of calories you burn in a day is known as your active metabolic rate (AMR). Rates vary from person to person, so it is crucial that you take the time to figure yours out — if you don't, you run the risk of consuming too many calories or even consuming too few.
Aside from determining how many calories you burn in a day, you also need to figure out what your physical activity level is. Determine which one of these descriptions best fits your day-to-day routine, then give yourself the appropriate score.
a. Sedentary Physical Activity Level
Do you have a desk job or do some other kind of work that keeps you in your chair for most of the day? If the answer is yes, your score is 1.1.
b. Light Physical Activity Level
Are you on your feet and walking around for at least half the day? Stay-at-home moms, salespeople, and doctors fall into this category. If this is you, your score is 1.2.
c. Moderate Physical Activity Level
If you're on the move pretty much all day, with a few limited periods of being sedentary, this is the level for you. People in this category include gardeners, carpenters, and mail carriers. If you're in this category, your score is 1.3.
d. High Physical Activity Level
Does your job require being constantly on the move, and does it entail significant amounts of manual labor? Construction workers, farm workers, and movers are among those who land in this category. If you're in this group, your score is 1.4.
Another element you need to calculate your AMR, is the number of calories you burn from exercise on an average day. The number of calories you burn during any exercise session depends on a few things, primarily your body weight.
Use this chart to help you determine the number of calories you burn from exercise on an average day according to your weight, the type of exercise you do, and its duration. You can also use a heart rate monitor to get an accurate number. Write the number down.
Now comes the math. If you don't know your BMR, it's influenced by a number of factors, including age, weight, height, gender, environmental temperature, and diet and exercise habits. Because of these varying factors, it's hard to pin down your BMR to the precise calorie, but we can get pretty close. Here are some formulas for figuring out your BMR. Get out your calculator and plug your numbers into whichever formula applies to you.
MALE: 66 + (6.23 × body weight in pounds) + (12.7 × height in inches) - (6.8 × age in years)
FEMALE: 655 + (4.3 × weight in pounds) + (4.7 × height in inches) - (4.7 × age in years)
Now you have all three numbers: your BMR, daily activity score, and exercise expenditure. Simply multiply your BMR by your daily activity score, and then add your exercise expenditure. Whatever you get from this final calculation is your magic number.