Technically, there is no magic number of calories we should all eat each day to lose weight. While most people can lose weight eating around 1,500 calories, you can assess your own personal caloric needs with a little math.

To estimate how many calories you should consume in order to maintain your weight, you'll need to do a little math. By using a simple formula called the Harris-Benedict principle, you can assess your basal metabolic rate -- also known as your BMR.
The calorie intake calculator provides an estimate of how many Calories (or kilojoules) you are needed for both men and women. 

To break this down into carbohydrates, protein, and fat - use the nutrient calculator. 
If you are looking to gain weight or muscle - use the bulking calculator. 
If you are pregnant and looking to gain the appropriate amount of weight needed for a healthy baby - see the pregnancy calorie calculator. 

(Then, to lose weight, you'll need to cut calories or burn extra calories and shoot for a level lower than the results you get with this formula.)

Calculate Your BMR
Your BMR is the amount of energy your body needs to function. We use about 60% of the calories we consume each day for basic bodily functions such as breathing. 

Other factors that influence your BMR are height, weight, age and sex.

Step one is to calculate your BMR with the following formula:

655 + (4.3 x weight in pounds) + (4.7 x height in inches) - (4.7 x age in years)

66 + (6.3 x weight in pounds) + (12.9 x height in inches) - (6.8 x age in years)

Please note that this formula applies only to adults.

Calculate Activity
Step two: In order to incorporate activity into your daily caloric needs, do the following calculation:

If you are sedentary : BMR x 20 percent

If you are lightly active: BMR x 30 percent 

If you are moderately active (You exercise most days a week.): BMR x 40 percent 

If you are very active (You exercise intensely on a daily basis or for prolonged periods.): BMR x 50 percent 

If you are extra active (You do hard labor or are in athletic training.): BMR x 60 percent 

The result of this formula will be the number of calories you can eat every day and maintain your current weightt. In order to lose weight, you'll need to take in fewer calories than this result.

As you lose weight, you can re-calculate the formula to assess your new BMR. 

Calories for Fat Loss 

Science tells us that 1 pound of fat is equal to 3500 calories, so a daily calorie deficit of 500 should result in 1 pound per week fat loss. In reality things don't quite work that efficiently!

When reducing calories:
Try not to lower your calorie intake by more than 1000 calories below maintenance. Doing so may invoke the bodies starvation response, which can lead to the Yo-yo dieting effect. 

Try to gradually lower calories. A sudden drop (such as 500 calories or more) can cause your metabolism to slow.
What happens when calories are too low?
1) Muscle mass is broken down for energy (catabolism). 
2) Metabolic rate will begin to drop (typically) after 3 days of very low calories - this is related to, and compounded by the loss of muscle mass. 
3) With very low calories you risk sluggishness, nutritional deficiencies, fatigue, and often irritability. You are completely set-up for a regain in fat if you suddenly return to your previous eating patterns. 
Lose Fat And Build Muscle?

Depending on your body type, it can be a very difficult balance trying to eat to burn fat, but retain or even build muscle. It's worth reading Tom Venuto's Burn the fat, feed the muscle (BFFM) for valuable insight on how to balance this. But realize that there is no single answer for everyone. It is a process of trial and error - but you need a starting point. 

If you are over age 35, then the ebook Fit Over 40 is an excellent read. It gives the cardio and nutrition regimes of over 50 different people who have achieved amazing fat loss and muscle tone.
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