If you are only concerned with calories, here is a link to a website that will allow you to calculate the approximate number of calories you should be consuming at 125 lbs.
http://www.caloriecontrol.org/calcalcs.html
To get there, calculate what your requirements are at 155 lbs and then reduce the daily calorie load by 500 calories. The closer you get to your target weight, the slower you will lose weight so to lose at a steady rate, you will also have to periodically recalculate your calorie requirements.
There are several things you need to consider before you go this route:
1) As someone who is probably still growing these numbers may be off by quite a bit for you. Your nutrient requirements should also be higher making a low calorie diet potentially harmful to your long term development.
2) This type of strategy rarely works. You might lose quite a bit of weight why you are focused on
counting calories, but most studies show that you will gain it all back (plus a little more) once you "go off the diet". Also, if you don't get enough exercise, your body will reduce your muscle mass. Reduced muscle mass = fewer calories burned = increased difficulty losing weight.
3) Why do you want to be at such a low weight? The only reason I can see is if you are wrestling or boxing. Even then, that is probably too light of a weight unless you have a very small frame. If you are looking to lose fat, focusing on your weight is not the best strategy. Instead, you should monitor your body fat % or your clothing measurements. These are much better indicators of fitness.
If your real goal is to improve your appearance by
losing body fat, your best bet is to change your lifestyle. The Mayo Clinic website (
http://www.mayoclinic.com) has some good tips on how to lose weight in their Weight Control section.
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