If you want an accurate idea of how many calories you burn in one day, you're going to need to do more than consult a simple chart based on generic criteria, such as gender and age, or gender and height. You will have to do a little math or visit websites that will do it for you. The serious approach to determining your daily use of calories entails calculating your resting metabolic rate and then combining that with all the activities you perform in a day.
Resting Metabolic Rate
You can start this process by finding your daily resting metabolic rate (RMR). This is the number of calories you would burn in 24 hours if you did nothing more than sit still. A scientific and time-tested method of calculating RMR is to use the Mifflin-St. Jeor equations, which were first described in 1990 in the "American Journal of Clinical Nutrition." The equations look like this:
Females: RMR = 10 x weight in kilograms + 6.25 x height in centimeters - 5 x age in years - 161
Males: RMR = 10 x weight in kilograms + 6.25 x height in centimeters - 5 x age in years + 5
You can easily calculate the value for yourself, but there are also websites that will do the work for you.
RMR Example
If you do the math yourself, here is an example you can follow. Assume Jane is 5 feet 3 inches tall, weighs 120 pounds and is 32 years old. This is how Jane would calculate her RMR:
1) Jane's weight in kilograms = 120 / 2.2 = 54.5
2) Jane's height in centimeters = 63 inches x 2.54 = 160
3) Jane's RMR = 10 x 54.5 + 6.25 x 160 - 5 x 32 - 161 = 1,224 calories
At this point, you have the calories Jane burns at rest in 24 hours. If you divide by 24, you would then have her hourly RMR = 1,224 / 24 = 51 calories per hour.
Metabolic Equivalent Task
Now, to figure the number of calories you use each day in activity, you will need to learn the metabolic equivalent task (MET) value for all of your activities. METs are numbers like 1.2, 6.7 or 12, which when multiplied by your hourly RMR give you the number of calories you use when doing that activity. Here is a sample of some MET values: working at a desk, 1.8 MET; doing household chores, 2.5 MET; running at 10 MPH, 10 MET.
Interpret this as "doing household chores uses 2.5 times as many calories as sitting still." There is a listing of more than 600 common activities at the University of South Carolina School of Public Health website.
Calculating Activity Calories
To calculate the total calories you use in one day, multiply your hourly RMR by the MET for your activities by the number of hours you spend in that activity. Let's do an example of this with Jane. Assume she spends her day in the following activities, with these MET values: sleeping, 0.9 MET x 8 hours = 7.2 MET; showering and grooming, 2.0 MET x 2 hours = 4 MET; working at her desk, 1.8 MET x 7.5 hours = 13.5 MET; doing household chores, 2.5 MET x 2 hours = 5 MET; running at 10 MPH, 10 MET x .5 hour = 5 MET; reading, talking, and eating, 1.5 MET x 3 hours = 4.5 MET; watching TV in bed, 1.0 MET x 1 hour = 1 MET.
Adding up her entire 24-hour day equals 40.2 MET, making her daily caloric burn = 40.2 X 51 = 2,050 calories. Note that is her total calories burned for one day, meaning that her activities added up to 2,050 - 1,224 = 826 calories above her resting energy expenditure.
Caveats
Remember that even though this method is much more accurate than using a standard calorie chart, it is still imperfect. All of the formulas and tables are based on statistical averages, so the further outside the norm you are in terms of height, weight or musculature, the less accurate they will be for you. Likewise, if you suffer from metabolic conditions such as diabetes, adrenal or thyroid problems, these calculations might also be less appropriate, but for most people, this process is a way to make a reasonably accurate estimate of the calories burned in a day.
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