There are 2 components in determining your daily caloric. The first is your resting metabolic rate (RMR). Your RMR is the amount of calories your body needs each day if you had no activity. 3 Common equations for finding your RMR are the Owen equation, the Mufflin Equation, and the Harris-Benedict Principal.
The second component is your activity level. Typically you choose the activity level for the type of life you lead and the formula tells you how many more calories to add to your RMR to determine your daily calorie needs.
The Harris-Benedict Equation:
For men: (6.24 x W) + (12.7 x H) - (6.76 x A) + 66
For women: (4.34 x W) + (4.7 x H) - (4.68 x A) + 655
The Mufflin Equation:
For men: (4.54 x W) + (15.9 x H) - (5 x A) + 5
For women: (4.54 x W) + (15.9 x H) - (5 x A) - 161
The Owen Equation:
For men: 879 + (4.64 x W)
For women: 795 + (3.27 x W)W = Weight in pounds
H = Height in inches
A = age in years
If you plug your information into the equations you will see that all the equations give you different results. Again, this is just an approximation to get you started. I recommend getting the average value for all 3.
Now that you have your RMR you need to add in calories for activity. Simply choose your activity level and multiply your RMR by the multiplier for that level
Very light activity - You spend most of the day sitting and rarely exercise.
Daily Calories Needed = RMR x 1.2
Light activity - You work in an office. Most days include some walking, bicycling, or stair-climbing, or you exercise at least 20 to 45 minutes once a week.
Daily Calories Needed = RMR x 1.375
Moderate activity - Your daily routine involves walking most of the day or includes lifting or you exercise at least 20 to 45 minutes about three times a week.
Daily Calories Needed = RMR x 1.55
Heavy activity - Your daily routine keeps you moving most of the time or includes some running, heavy lifting, or swimming; or you exercise at least 20 to 45 minutes every day.
Daily Calories Needed = RMR x 1.725
Exceptional activity - You're unstoppable. You're training daily for an athletic event, or you're a professional athlete with a strenuous schedule.
Daily Calories Needed = RMR x 1.9
A majority of people fall into one of the first 2 categories. When you pick your category, be honest with yourself. Don’t pick the category you plan on being at, pick the one you are at right now.
Once you know how many calories you need each day to maintain your weight, you can now determine how many calories to eat in order to lose weight. Each pound is worth 3500 calories. If you eat 500 calories less than you need each day, you should lose 1 pound each week. Eat 1000 less each day and you can lose 2 pounds each week. Eat 2000 less each day and watch out; if you restrict yourself too much, your body will react and slow your metabolism down. If that happens, your RMR will go down and you might not lose the weight you wanted.