The number of calories you need for energy is determined by four factors: your age, your gender, your size and, most important, your level of activity. If you are a teenager, expect to need more calories as you get older. Conversely, if you are an adult, your body will require less calories as you age.
Base Point
The base point of your caloric needs is your resting metabolic rate, or RMR -- what your body will burn just to keep your vital organs functioning. From this point, the Institute of Medicine Dietary Reference Intakes adds increments of average activity to arrive at your estimated energy requirement, or EER. A sedentary individual is someone who performs only those activities necessary for day-to-day life. If you are moderately active, you expend the amount of energy necessary to walk up to three miles per day at a moderate pace in addition to daily activities. An active person burns the energy to required to walk more than three miles per day or the equivalent.
Usage
Most of the calories you consume feed your muscles, according to the Marquette General Health System -- approximately 75 percent. If you are trim with little body fat, you therefore need more calories than someone who is overweight. The simple act of eating also burns calories, about 10 percent of what you consume. Protein requires the most calories to digest.
Other Factors
Beyond your EERs, figuring out the exact number of calories you need for energy requires a bit of math, according to the American Council on Exercise. The easiest way to do this uses your EER in connection with your weight and gender. If you are a woman and you weigh 125 lbs., multiply 125 by 12.5 if you are sedentary, 14.5 if you are moderately active or 16 if you are very active. For instance, 125 times 14.5 equals about 1,812 calories a day to maintain your energy level if you are moderately active. Men should multiply their weight by 14.5 for a sedentary life style, 16.5 for a moderately active lifestyle and 18 if they are very active.
Calculating Activities
You can also calculate additional calories you might need for specific activities that fall outside the realm of your normal pattern. For instance, if you are a man and you weigh 180 lbs. and you spend 15 minutes chopping wood on a Saturday afternoon, start by multiplying 0.9 times your weight in kgs, with one kg equaling 2.2 lbs. of body weight. Your kg weight would be about 82. Multiplying that by 0.9 equals 7.4. Now multiply 7.4 by the number of minutes you spent chopping wood -- 15. You needed almost 111 more calories to have chopped that wood. Different activities expend a wide range of calories. While the calorie factor for chopping wood is 0.9, folding the laundry expends only 0.4.
Warning
Starvation diets rarely work because of the way your body reacts to the calories you give it, according to the Marquette General Health System. When you consume less calories than your body needs to maintain its resting metabolic rate, it begins to conserve the calories that you do provide by lowering your RMR even more.
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