I guess you know already that 1 gm of carbohydrate or protein produce 4 calories, while 1 gm of fat produce 9 calories. So that is why if you eat a food which weighs 100 gm, and full of carbohydrate, you are taking 400 calories (4 X 100). Now if you take a food which is fully made of fat, like vegetable oil, then 100 gm means 900 calories. So if you eat 450 gm (almost equal to one pound) of food (containing fat only), you war taking 450 X 9 = 4050 calories. So you see 1 pound of fat is producing 4050 calories. So to lose one pound of fat you have to burn 4050 calories. A little correction. Though we read and know that 1 gm of fat means 9 calories, it is not exactly 9, rather around 8.6. So if you multiply 450gm (which is one pound) then you will get around 3500. So finally you now know that to lose one pound of fat you have to burn 3500 calories.
3500 calories burn one pound !
If today you burn 3500 calories you will not lose one pound of fat. Why ? Let me explain the catch here. Suppose you take food worth of 3500 calories, and burn also 3500 calories, then the net calories balance is zero. So you will lose nothing, not a single gram (at least theoretically !). Ok, I know you are screaming, "Hey I am not going to eat 3500 calories". That's truth, you are not. Suppose you will eat around 1750 calories, so net gain is 3500 -1750= 1750, half of what you burn. So you should lose half a pound in that day.
I hope you know that most safest weight loss program is one which goes slow. Health professionals always recommend slow weight loss programs. Gradual weight loss is certainly the most effective approach. If you lose weight gradually, like one-half pound per week, you will find it easy, you can easily maintain it for long term. This is alos truth that many people like to see some reasult at the begining. It is not a bad approach, if you see the reasult early, you get motivated too. So may be you can set a goal of losing one pound of fat a week initially, may be just for few weeks, then you can switch to one-half pound per week.
One pound of fat lose per week
So, to lose one pound of fat in each week, what you have to do? First check how many calories you can spend a day. If your weight is higher than normal, then you can burn a high number of calories, by spending some time in walking or with other physical workout. Suppose you can spend 3000 calories a day. Now let's see how much calories you are eating a day . If you weigh more, it is very likely that you have the habit of eating more. But if you want to lose fat, you have to eat less ( otherwise you have spend more time on physical workout, either way it will work).
So try to limit your food intake to 2500 calories. So finally you are earning a net gain of 500 calories (3000 -2500) on each day. Add this for a week, 7X 500, 3500 calories, net gain, which means you will lose one pound a week. Now if you can not lspend 3000 calories but only 2500 calories a day, what you will do! Don't worry still you can shred off one pound a week. Only that you have to eat less. Since you are spending 2500 calories a day, you should now eat 2000 at a rate of 2000 calories, because 2500 -2000 is again giving you a net loss of 500 calories.
More realistic goal - two pounds a month.
To lose one pound a week you have to eat less and engage in more physical workout. Truth is, for a overweight person ( we are dealing here with overweight person, aren't we?, otherwise why you have to lose weight ! ) eating less and exercising more is not really very easy. A goal of around two pounds fat lose a month, is a better option. Just two pounds a month means around 100 pounds a year. It is better to look at things at wider perspective. Getting in shape in two three months is achievable but very difficult to maintain. Rather you should choose a plan which work on long term basis. So for two month a pound lose - you need to take 250 fewer calories a day, which should not be difficult at the beginning. Don't forget you are now on the other side of the coin, probably taking few hundred extra calories each day, which caused you gaining weight in last months or years.
Losing weight means burning less calories
Since the amount of calories body spends, depends on the weight of the body (more weight means more spend), once you start losing weight, your body will start to spend less calories at rest time. If you want to keep your goal same, then you need to adjust your calories intake, meaning you have to take fewer calories. So day by day it would be difficult. On the side side, the slimmer you will become, you will feel less hungry, so your body will demand less too. So the catch is you have to recalculate you food intake and physical burnout once you start losing weight, every now and then.