• 1 gram of carbohydrate =4 calories
•1 gram of protein = 4 calories
• 1 gram of fat =9 calories

The amount of calories our bodies need to do daily activity is called Calorie Maintenance Level (CML) .
To lose weight your body must have less calories to utilise which would making it burn your fat stores for energy. Therefore, a caloric deficit is what causes weight loss. To attain calorie deficit you must do the following:
1. Consume Less Calories (Diet) one of the most important way to lose weight is by consuming less total calories. If your body needs 3500 calories per day (just an example), then you should take about 3000 calories per day, therefore supplying your body with less calories than it needs to maintain your current weight. Doing this consistently will cause weight loss. 
2. Burn More Calories (Exercise) The second way to attain calorie deficit is by burning more calories than your CML. If you make your body burn additional calories each day through some form of exercise, there would then be a caloric deficit. One thing about exercise is that it raises your metabolic rate not only while you’re exercising, your metabolism takes a while to return to its normal pace. It continues to function at a higher level and your body burns an increased number of calories for about two hours after you’ve stopped exercising.
To know how many calories you need or burn in a day, i suggest that you use Basal Metabolic Rate (BMR)
BMR: is the amount of energy our body usually needs to function at rest. It differs from human to human and accounts for about 60 to 70 % calories burnt in a day.
The best method of estimating your basal metabolic rate is the Harris-Benedict formula:
• Adult male: 66 + (6.3 x body weight in lbs.) + (12.9 x height in inches) - (6.8 x age in years)
• Adult female: 655 + (4.3 x weight in lbs.) + (4.7 x height in inches) - (4.7 x age in years)
 
The BMR only represent resting energy expenditure, an adjustment must be made to reflect your activity level. This is done by multiplying your BMR by an activity factor (McArdle et al 1996). Note that the following activity factors also take into account the thermic effect of food Activity.

1 Pound Of Fat = 3500 Calories  
Meaning that for every 3500 extra calories that you consume that your body doesn’t burn, you will GAIN 1 pound of fat. At the same time, it also means that for every 3500 extra calories that your body does burn, you will LOSE 1 pound of fat.
By eliminating 500 calories a day, you will lose one pound in a week. So, if your daily caloric need is X to maintain your weight, subtract 500 from that, and you’ll have the number of calories you can consume each day and still lose a pound in one week.
The Ideal Weight Loss Rate is 1-2 Pounds Lost Per Week, even though we all want to lose weight faster than that. Everyone does. I personally wish I could drop 20 pounds by next week, but i can’t. Not only that, but attempting to lose weight faster than the above recommendation can actually be quite harmful to your health. 1-2 pounds may not sound like much, but it is. In a month and you could lose 4-8 pounds and in a year, 50-100 pounds. It may not be as fast as you wish it could, but long term… it’s really not that bad at all. 

 check http://trimandtrendy.blogetery.com for full details