The cost of the federal food stamp program will increase 43 percent from last fiscal year to next, if President Carter's budget estimates are correct.

The program, which cost $6.8 billion last year, will cost $9.7 billion in fiscal 1981.

It will serve an estimated 20 million people that year, about one American in 11.

Its phenomenal growth rate -- it is now second in size amont welfare programs only to Medicaid, federally subsidized medical care for the poor, and is much larger than the federal share of any of the cash welfare programs -- is seriously troubling Congress.

Rising food prices and an expected increase in unemployment are the two main reasons for the program's growth. But conservatives also believe that the rules are so loosely drawn that many people with fairly large income can and do get stamps.

In 1977, the last time it liberalized the eligibility rules, Congress also moved to control the stamps program by putting a "cap" on its annual cost. This year that cap is $6.2 billion, or about $2.5 billion short of the amount officials estimate they will need to give stamps to all who apply and are eligible. For fiscal 1981 the cap will be about $6.2 billion under persent law, some $3.5 billion less than the estimated full cost.

The president wants the cap lifted. Otherwise, the program will run out of money in a few months. The Senate last year voted to remove the cap. Many House conservatives would be willing to raise the cap, but not lift it altogether, for fear of losing all control.

Sen. George McGovern (D-S.D.) and Bob Dole (R-Kan.), who have worked in tandem in recent years to improve the program, said last week that it is a major reason for reduced malnutrition among the poor of the United States. They oppose the cap.

McGovern cited a 1977 survey by the Field Foundation, which credited food stamps for the fact that "there are far fewer grossly malnourished people in this country today than there were 10 years ago."

Four years ago, when it reauthorized the program, first created in the Kennedy-Johnson administration, Congress tightened up eligibility rules. Before then, people were eligible for benefits if their net income was up to 125 percent of the poverty level. This was lowered to 100 percent.

The eligibility change, should have reduced participation, but it was counteracted, according the Agriculture Department and Senate aides, by another changes: elimination of the "purchase requirement," as of 1979.

Under this requirement, a family whose income level made it eligible for $60 in free stamps had to buy an added $40 with its own money, ending up with stamps worth $100 for a cost of $40.

The idea was to make sure the family spent some of its own cash on food.

This requirement acted as a brake on participation by people who were eligible for a small amount of stamps and didn't want to put up a large amount of their own cash to buy them; and on people with so little income they could not affor the cash payments.

Under the 1977 amendments, the purchase requirement was eliminated. A family eligible for $60 in free stamps get them free.

One result, predicted at the time, had been addition of several million people to the rolls.

Other factors that have increased program costs, according to Bob Greenstein, administrator of the Agriculture Department food and Nutrition services:

Inflation, because under the law food stamp allotments rise automatically with food prices, so that a house hold can always theoretically afford a so-called "thrifty" diet. In 1976, according to a Greenstein aide, the cost of a complete monthly "thirfty" budget for a family of four without other income -- containing enough food to feed the whole family for a month -- was $166 for a month.

Today, to buy the same items costs $209 a month, and that's how much in stamps such a family would get. The cost is recomputed each half-year.

The $209 monthly free stamps provides 57 cents per meal for each family member.

Unemployment, and therefore participation the the stamp program, has risen. It had been projected at about 5.1 percent, but is now higher and is expected by Oct. 1 to be about 7.5 percent.

New entrants to the program recently have been among the poorest in society, largely from rural areas and the elderly. This means extra costs, because poorer people get more stamps each month.

A Greenstein aide said program statistics belie the argument that well-to-do or not-so-badly-off people get a lot of benefits.

For example, a family of four is eleigible for benefits only if its income is below the poverty line ($7,150 a year), but income used for certain types of expenses, such as child care, work expenses and extra-high housing costs isn't counted.

As a result, a family of four could have gross income of up to $11,412 and still get stamps, though this isn't common.

Half of those on the rolls have gross incomes, before deductions, of $3,200 a year or less. Some 97 percent of all recipient families have gross income below $9,000. Half the recipients are children under 18, and another 30 percent are women in husbandsless homes, disabled persons and the elderly.

Conservative critics of the program however, believe there is a bit too much "give" in the rules.

Hyde Murray, an agriculture expert and GOP House aide, said the poverty-line cutoff on eligibility is misleading. It doesn't count in-kind benefits like Midicaid, housing subsides, school lunches; it doesn't count certain types of income used for childcare (up to $90 a month), extra-high rent, medical and work outlays and up to $75 a month for "general" expenses.

All these can be deducted before seeing if you meet the $7,150 standard. And you can own a house and a car, plus a tractor or truck used in your work, and still be eligible.