3 months might be a long time-frame for you, but this is where the scientific part comes into it.
It's universally accepted that there's 3500 calories per pound of fat. That means, for every 3500 calories you over-eat (based on your BMR; I'll get into this in more detail in a moment), you'll add on a pound of fat, and likewise, lose a pound of fat for every 3500 deduction based on your BMR.
BMR stands for basal metabolic rate, which is the amount of calories your body needs in a single day to remain in the same shape without doing anything (e.g. in comatose). It's calculated based on your height, weight and age.
This is usually adjusted by being multiplied by a factor of 1.2x - 1.9x (I believe), to account for the kind of lifestyle you lead; e.g. sedentary where the only real exercise you might do is walking around the office and your house).
For example. I'm 17, I weigh 205lbs and I'm 6"1'. My BMR is 2155 calories (the amount I would need to remain in the same shape if I was in comatose), which is then multiplied by 1.2 (as I'm a rather sedentary person), to give a figure of 2585 calories.
I can eat 2585 calories a day (adjusted by perhaps 2 - 5% to account for calorie averages on packets and what not, just something I personally do), without putting on or losing any weight.
What I'm trying to illustrate (aside from imparting a little scientific basis behind weight loss) is that there's a maximum to how much weight you can lose in a given period of time, especially if you want to be safe.
Personally, at my calorie requirements, I wouldn't go anywhere under 1300 calories a day to be safe. So let's say I consume 1300 calories a day, I've now got a deficit of 1285 calories per day. I need a loss of 3500 calories, per lb of fat, which means that I'm losing 1lb ever 2.7 days, with a total loss of 2.57lbs per week.
Personally, I want to lose an additional 15lbs. On the basis of 2.57lbs of weight loss per week, it'll be at minimum 6 weeks before I reach my goal.
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That's a basic breakdown of the science and mathematics behind losing weight, but by no means a thorough look into the subject. It's nice to think it's that easy, but adjustments need to be made when you factor into account;
- Muscle loss. While you'll be burning fat, there's also a significant potential to lose muscle mass too, especially if you don't intake enough protein and do some mild form of strength training.
- BMR calculations are universally accepted, but there are three different methods of calculating adjustments for lifestyle types, each one +/- 50 to 75 calories or so. Make sure whatever calculation you use, you use it consistently.
- As your weight drops, your BMR will also drop, and thus the speed at which you can lose weight.
- Etc.
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Resources for the OP, and anyone else who wants them;
- http://www.bmi-calculator.net/bmr-calculator/(for calculating your BMR, make sure to multiply your result accordingly, details of how to do so on the next page).
- http://forum.bodybuilding.com/ (hands down the net's most important and well organized forum for fitness training in general. Is meant to be specifically for BodyBuilders, but cutting (the slangish term for burning fat) is a key part of a BodyBuilder's regimen and as such there's a lot of great details on how to lose weight)
OP, I really wish you luck on your weight loss journey! I don't know how much you have want to lose, but whatever it is, I hope you don't falter
. A lot of people go into weight loss thinking they can control the majority of it by exercise, but in my opinion, weight-loss is at least 80% if not more diet based.
Again goodluck!
