We are just emerging from the fattest time of the year. The so-called "fat months" usually start in December, with the holiday feasts and first storms of the winter. Most of us tend to eat and drink too much during the holidays, and then use the bad weather as an excuse to stay indoors watching TV or reading a book (while munching high-calorie snacks) rather than "working off" the extra calories in some sort of outdoor activity.

Since this winter has been particularly harsh in many parts of the country, we suspect that many people will find that they need even more shaping up than usual. All too often, the temptation is to urge a more logical mathematical approach. And since many of you have just been through the annual fast-time budget crunch, balancing off caloric intake versus expenditure should have a familiar ring. It goes someithing like this.

Let's suppose that you are a relatively inactive housewife in your early 50s. You weigh 145 pounds before you go on your vacation in August. The first step in planning your calorie budget for the next few months is to determine how many calories a day you are now consuming. To do this, keep a careful record of what and how much you eat for a week. Using a calorie counter, you can get a good estimate of the number of calories you are taking in each day. Let's suppose it averages out to 2,200 calories a day.

Now you need to figure out how many calories you expend in a typical day. It might go something like this: eight hours of routine housework, walking around the house, etc., 1.9 calories per minute or a total of 912; one hour washing, dressing, etc., two calories per minute or 120; one hour shopping, driving, etc., 1.8 calories per minute or 108; six hours of sitting, reading, etc., 1.3 calories per minute or 468; eight hours in bed, .8 per minute or 384. This adds up to a total of almost 2,000 calories expended each day.

Since your intake is 200 calories more than your expenditure, and there are 3,500 calories in a pound of fat, you can expect to gain nearly two pounds a month, or an eight-pound weight gain during the winter months.

While losing these extra pounds may seem harder than putting them on, it is also a matter of mathematical logic. If you eliminate those 200 extra calories a day by, say, foregoing five teaspoons of sugar in coffee and tea and substituting an apple for ice cream as your dessert, you would be in caloric balance, burning up as many calories as you consume. But if you reduce your caloric intake by 500 more calories a day and maintain your present level of activity, you'll lose about a pound a week, meaning that you'll reach your goal of 125 pounds in 20 weeks.

As an alternative to cutting down 500 more calories, you could add two hours of walking to your daily activity, which will burn up 240 of those calories. If you decide to take off only a pound a week, you'll need to reduce your caloric intake by 260 calories instead of 500. And if you decide to do both - walk two hours a day and consume only 1,500 calories, which is still enough to maintain proper nutrition - you'll lose 20 pounds in about 13 weeks.

Of course, the more vigorous the physical activity, the more calories the activity consumes. Bicyling, for example, can burn up about 100 calories in 15 minutes. Swimming for an hour consumes nearly 600 calories, depending upon the stroke used and the temperature of the water. Twenty minutes of jogging will burn up 300 to 350 calories.

Like balancing your household budget, every little bit adds up. Walking up a couple of flights of stairs instead of taking the elevator; walking those four or five blocks to the store instead of driving; even pacing back and forth while you talk on the telephone will consume more calories than sitting watching TV or reading a novel. By the same token, a glass of beer, a handful of peanuts or potato chips, acouple of cookies or a donut with your mid-morning coffee, all add unnecessary calories.

Maintaing your proper weight, just like living on a budget, does not need to be a constant struggle. Instead, it's all a matter of knowing how much you have to psend, and doing it with a bit of common sense.