Managing calories is much like balancing your checkbook. If you maintain a balance between calories in and out, you'll maintain your current weight. If you consistently consume more calories of food energy than your body uses, you'll create a surplus, which is stored as body fat. If you consistently burn more calories on activity than you take in eating, you'll create a deficit, resulting in weight loss.

Some people—a very small minority—have physical problems that interfere with their ability to burn calories properly. These people should consult their doctor about weight issues. But for most people, weight loss and gain are simply a matter of calorie balancing.

There are 3,500 calories in a pound of body fat. This means that to lose a pound a week, you have to burn 500 more calories a day than you take in (seven days x 500 calories = 3,500 calories). To gain a pound a week, you've got to take in a surplus of 500 calories a day.

Determining your calorie needs
On average, women need 10–11 calories per pound per day to stay at their current weight. Men need 12–14 calories per pound per day. (People with more muscle can use the higher end of these ranges.)

Using these figures, you can estimate how many calories to add or cut each day to get to your goal weight.

Here's how to do these calculations yourself:

1. For women, multiply your current weight by 10 if you're nonmuscular, 10.5 if you're of average musculature, and 11 if you're highly muscular. For men, multiply your current weight by 12 if you're nonmuscular, 13 if you're of average musculature, and 14 if you're highly muscular.
2. Then, for women, multiply your goal weight by 10 if you're nonmuscular, 10.5 of you're of average musculature, and 11 if you're highly muscular. For men, multiply your goal weight by 12 if you're nonmuscular, 13 if you're of average musculature, and 14 if you're highly muscular.
3. Subtract the number you got in step #2 from the number you got in step #1. Write down the difference, but no more than 500 calories, which corresponds to a weight loss or gain of 1 pound a week. If you want to lose weight, this number represents the daily calorie deficit you consistently need to achieve until you reach your goal weight. If you want to gain weight, you need to take in this much of a daily calorie surplus to get there.

Here's an example: Jill weighs 170 pounds. Her goal weight is 135.

To maintain her current weight, she needs to consume 1,870 calories per day (170 x 11 = 1,870). To maintain her goal weight, she would need to consume 1,485 calories per day (135 x 11 = 1,485).

To reach her goal weight, she needs to achieve consistently an average daily deficit of 385 calories (1,870 - 1,485 = 385).

This can be done by reducing the number of calories in her diet, increasing the calories she burns through activity, or both.

Balance your calories over time
Highly restrictive diets often are based on creating a perfect balance, on a daily basis, between calories consumed and calories spent. This usually leads to failure because it goes against human nature. People's eating patterns and activity levels fluctuate from day to day. Everybody overeats occasionally, and has days when they're tied to their desks or feel too lazy to exercise. One key to healthy weight loss is to balance your calories over time. Your eating and exercise plans can then be flexible enough to accommodate all the situations encountered in real life—holidays, vacations, illnesses, or working extra hours. Aim for the right balance over several days.