That's a loaded question!
There are many factors that determine your caloric need including your age, height, gender, weight, amount of lean body mass (muscle), and activity level. Fortunately there are formulas that can help you figure out how much you should be eating daily to manage a healthy weight. In one way or another, they drive the more reputable calorie calculators you'll find online. While not as accurate as direct testing, the formulas can be used to quickly and conveniently guide you to the number of calories that best suit your needs. Keep in mind that it's important to know which formula is being used, as well as how the formula works, so that you understand the answer you're given.
In 1918, two biochemists, J. Arthur Harris and Francis G. Benedict published a research paper entitled, "A Biometric Study of Human Basal Metabolism". From the results of this research, Harris and Benedict developed a revolutionary formula that has been used for decades to help determine the caloric need of individuals especially when physical assessment is not possible. Your Basal Metabolic Rate (BMR) is a measure of the amount of calories your body uses (burns) each day for all autonomic functions such as circulation, breathing, and digestion. Once you have calculated that number, use the activity multiplier to determine your final figure.
Calculate it manually:
English BMR Formula
Women: BMR = 655 + ( 4.35 x weight in pounds ) + ( 4.7 x height in inches ) - ( 4.7 x age in years )
Men: BMR = 66 + ( 6.23 x weight in pounds ) + ( 12.7 x height in inches ) - ( 6.8 x age in year )
Metric BMR Formula
Women: BMR = 655 + ( 9.6 x weight in kilos ) + ( 1.8 x height in cm ) - ( 4.7 x age in years )
Men: BMR = 66 + ( 13.7 x weight in kilos ) + ( 5 x height in cm ) - ( 6.8 x age in years )
Or use a calculator.
Next, use the activity multiplier to calculate your Total Daily Energy Expenditure (TDEE):
* If you are sedentary: BMR x 1.2
* If you are lightly active: BMR x 1.375
* If you are moderately active (you exercise most days of the week): BMR x 1.55
* If you are very active (you exercise intensely on a daily basis or for prolonged periods): BMR x 1.725
* If you are extra active (you do hard labor or are in athletic training): BMR x 1.9
To lose weight, you need to create a deficit of 500 - 1000 daily calories. Optimally, this should be done with a combination of diet and exercise. You might also try inserting your target weight into the formula and using that figure for your daily intake. Do keep in mind that you may need to adjust the number as your muscle mass and intensity of exercise increases.
To your fitness success!