You need enough calories to cove your energy expenditure throughout the day. If you want to lose weight than you need to burn more calories than you consume, it's as simple as that. But how many calories do you burn throughout the day is the question. First you need to determine your BMR (basal metabolic rate) which for men is 1 calorie per kilogram of body weight per hour and 0.9 for men. A kilogram is 2.2 pounds. So a for a 150 lb. male it would be 68 cal/hr and for a 150 lb female it would be 61 cal/hr. You simply then multiply that number by the hours in a day (24). So for the 150 lb. male it would be 1632 cal/day and for the female it would be 1464 calories per day. This is only the first half of the equation, now you need to take into account your activity level. The different levels are in order as follows:
Sedentary: (Men 25-40%, Women 25-35%)- You sit around most of the day.
Light Activity:(Men 50-70%, Women 40-60%)- Your activities are as intense as house cleaning/table tennis.
Moderate Activity:(Men 65-80%, Women 50-70%)- 1 hr. of fast walking 4-5 times a week.
Heavy Activity:(Men 90-120%, Women 80-100%)- Physical labor like a roofer, soccer, basketball.
Exceptional Activity:(Men 130-145%, Women 110-130%)- Many hours a day of intense training like phelps.
So you find your activity level and take the low percentage and the high percentage and multiply each by your calories per day. So for the 150 lb. male that is moderately active it would be 1632 x 65%=1060 and 1632 x 80%= 1306 calories per day. So that 150 pound guy needs between 1060 and 1306 calories for his activities in a day. Now you add the BMR to the activity needs. 1632+1060=2692 and 1632+1306= 2938. So his range is 2692 to 2938 calories a day.