The number of calories you need each day decreases as you age, assuming all other factors are equal. An accurate estimation of your daily calorie needs therefore requires you to take your age into account. Additional factors that affect your calorie requirements include your height, weight, gender and activity level.
Basal Metabolic Rate
Basal metabolic rate, or BMR, is the minimum number of calories you can possibly burn each day. Your BMR does not include the calories you need to perform any activity. This includes the calories you need to digest food and regulate your body temperature.
Activity
Your activity level also has a significant effect on your calorie requirements. Your activity factor is one if you're at complete rest, and a sedentary lifestyle has an activity factor of 1.2. An activity factor of 1.375 applies to people who engage in light exercise two or three times per week. An activity factor of 1.55 applies to people who are moderately active, and an activity factor 1.725 is for people who engage in heavy exercise up to six times per week.
Formula
Weill Cornell Medical College provides the following equation to calculate the BMR for men: BMR = (5.003 x Height) + (13.75 x Weight) - (6.775 x Age) + 66.5. The corresponding equation for women is BMR = (1.850 x Height) + (9.563 x Weight) - (4.676 x Age) + 655.1. These equations require your weight in kilograms and your height in centimeters. They provide your BMR in units of calories per day.
Limitations
These equations do not consider your body fat percentage, and are therefore most accurate if you have an average body composition. Muscle requires more energy to sustain than fat, so these equations will underestimate your BMR if your body fat percent is below average. Similarly, these equations will tend to overestimate your BMR if your body fat percentage is above average.
Calculations
Multiply your BMR by your activity factor to obtain the number of calories you need each day. Assume you're a 56 year-old man for this example. Your height is 186 cm and your weight is 65 kg. Your BMR is therefore (5.003 x 186) + (13.75 x 65) - (6.775 x 56) + 66.5 = 1,511 calories per day. Assume you have a sedentary lifestyle for this example. Your calorie requirements are therefore 1,511 x 1.2 = 1,814 calories per day.
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