Some time tomorrow, Gladys Curtis, an 81-year-old retired bookkeeper and businesswoman, expects to receive a brown evelope containing her Social Security check for July. The payment -- usually $229 a month -- will be $254 this time, thanks to an 11.2-percent annual cost of living increase.

But while Curtis welcomes the extra $25, she said it is only about half of what she needs to pay a $46 rent increase effective last month for her small apartment in a downtown retirement home.

Curtis is among nearly 235,000 people in metropolitan Washington and 36 million people nationwide -- or roughly one of every seven Americans -- who will receive more than $145 billion this year in retirement, disability and survivors benefits. In 1982, the benefits paid out will climb to $164 billion, under current Reagan administration budget proposals.

For three-fifths of all beneficiaries who are 65 or older, Social Security benefits repesent 50 percent or more of their total income. For 16 percent, the benefits are their only source of income. For them, living expenses have raced for ahead of the annual benefit increases. Although the increases are vital, they still are not enough to match the steadily rising prices of food, housing and other essentials.

For a minority, however, the income they receive from Social Security goes not for necessities but for little extras. Their monthly check is their earned payback for years of work and contributions to the Social Security fund; but investments or other sources of income are sustaining them in retirement, not Social Security alone.

On Capitol Hill, some members of Congress -- looking for ways to restrain the growing costs of Social Security -- are locked in debate over whether the automatic system of increases approved nine years ago should be replaced with a new merchanism that costs less and is more in line with the generally smaller increases in income paid to full-time wage earners in recent years.

In the spring of 1980, Gladys Curtis sold her duplex in surburban Philadelphia and made plans to move to Washington, where she would be closer to her two physician granddaughters. With some investment income and Social Security benefits of $212.76 a month, Curtis felt she could afford an apartment at the retirement home.

She paid the home's $16,500 fee last July to cover a lifetime of services, moved into an efficiency apartment that overlooks a roof top and a parking lot, and began paying a monthly rent of $420. In less then 12 months, that rent grew to $506 a month.

"I was shocked when they sent me a notice that my rent would be raised $40 last October," Curtis, a window for 32 years, said recently. "I hadn't completely gotten over the first increase when I got another notice this year that they were going to raise my rent another $46 on June 1."

To pay the increased rent and an $11-a-month increase in her Blue Cross-Blue Shield coverage, Curtis said she has had to pare her budget continually. She said she is now down to the essential -- $15 a month for her telephone, about $50 for food.

"My budget doesn't allow for much recreation," said Curtis. "I just want to be careful not to spend up all of my assests. I want to keep enough so that I can be buried at home in Pittsburgh."

Jospeh A. Loftus, a former journalist with a retirement income of $50,000 a year, said the $50 Social Security increase he will receive will enable him and his wife, Mary, to have dinner one additional time a month at La Miche, their favorite French restaurant, in Bethesda.

For Loftus, who lives in upper Northwest Washington, the monthly $438.50 Social Security check has been extra income, to be added to the $376 monthly retirement pension he gets for his 25 years as a reporter for the New York Times and a hefty income from security investments.

"I'm not dependent on Social Security for my life style," said Loftus, 73. "But my pension doesn't provide for cost-of-living adjustments. My Social Security does. And that has been an important factor in our efforts to keep with up inflation."

As a tractor-trailer driver, Thomas Hopkins earned $2,052 a month in April 1980 for himself, his wife, Patricia, and their four children. But surgery to remove a blood clot from his brain and a subsequent stroke left Hopkins unable to work. Two months later, the family income had dropped to $1,205 a month, which included a $370 Social Security disability check, $179 a month in food stamps, and $164 a week that Patricia Hopkins earned when she took a job as a telephone operator.

In July, Thomas Hopkins' Social Security benefits will increase by $41 to $411. The increase, said Hopkins, now 46, will largely go to pay expenses he has already incurred. In May the rent on his three-bedroom town house in Alexandria went up $20. His electricity bill has jumped by $30 because he must use his air-conditioning in an effort to ward off a possible heat stroke and other health problems.

"We're just squeezing by from month to month," said Hopkins, who added that some of his creditors are demanding payment. "The doctor says I can't go back to work. It's really hard to get school clothes for the children. And after I get the increase they [social workers] say I probably will get less food stamps."

"The basic idea of the social security system was 'income insurance,''' said Robert Ball, former commissioner of the Social Security Administration. "The idea was to allow people to pay into a fund during their working years that they could draw from to cover part of the income lost through retirement, disability or death."

Before 1972 Congress had not provided for automatic cost-of-living adjustments for Social Security recipients. Increases had to be voted by Congress, and often were -- during election years. That year, however, Congress established a mechanism for automatic increases for both taxes and benefits based on wages and the cost of living.

"The mechanism established by Congress in 1972 was designed to keep beneficiaries even with the cost of living," Ball said. "Although most estimates are that the increases have fulfilled their purpose, there is a lag in the impact of the raised benefits because people frequently have already suffered a loss by the time they get the additional money."

"Another way to look at the annual increase is that it'a a little higher to make up for any gap between the benefits and price increases," said Robert J. Myers, the Social Security Administration's deputy commissioner for programs.

"The system gives a rough approximation of social justice," he said. "The amounts paid in benefits are not based on opinion, but on the contributions the person has paid into the system.It is not a needs-test program. A man with a billion dollars may not need Social Security, but he is just as much entitled as anyone else."

Largely as a result of the automatic increases over the last nine years, the number of elderly people below federal poverty standards has dropped dramatically over the last two decades.

In 1959, about 35.2 percent of all persons 65 and over lived in proverty, according to the Social Security Administration. By 1979, the figure had been reduced to about 15 per cent.

One issue in the Capitol Hill debate is whether Social Security benefits should continue to be based on automatic price increases, using the Consumer Price Idex as a guide, or whether the increases should be tied to wage increases -- or a combination of the two.

"The most common proposal favors a combination of wage and price indexing," Meyers said. The increase would be based on the lesser increase in prices or wages. If this year's increase had been based on wages, the increase would amount to only 9 percent, rather than 11.2 percent."

Each month, Minnie L. McCullough, 79, receives a blue-green check for $331 -- her payment after more than four decades of work as a cab driver and as a registered nurse.

Out of the Social Security retirement check, her only source of income, she pays the $125 rent on her little apartment in Takoma Park. Her health insurance costs $11.49 a month and she spends another $50 for groceries. hShe buys gasoline -- about $20 worth a month -- and pays for the maintenance of her 1973 Chevy Nova. Occasionally she buys a new dress.

McCullough hopes that with the additional $37 she expects to receive in her check there will be enough money left to eat out once or twice more a month at the Hot Shoppes Cafeteria in the Prince George's Plaza shopping center, where her favorite fare is a serving of calves liver and a slice of strawberry pie for $3.41.

"An 11-percent increase isn't a million dollars," McCullough said, "but I believe I'm a little better off with it than without it."

When her husband died of a heart attack eight years ago, Evelyn M. Scott and her five small children were left with her then-parttime income as a registered nurse and a Social Security check for about $390. Since then, four of the children have gone on to college.

Without the Social Security survivors benefits and the annual increases that have pushed the monthly checks up to $1,000, Scott, 48 -- who now earns $20,000 as a full-time nurse -- said the family would not have been able to maintain its middle-class life style. Next week, the Scotts' check will go up another $112.

"I've been very lucky," said Scott, a resident of Elkton, Md. "We didn't know how we were going to make it with the cost of food, clothes and things like insurance going up constantly. When the kids were ready for college, they didn't qualify for financial aid because the family's income was too high."

Still, she said, with Social Security, money from her husband's life insurance policy, and money the children earned from part-time jobs, the bills at college and at home have always been paid.

Francis Holsinger was a classroom teacher in eastern Maryland for eight years, a principal for 29 and a county supervisor of secondary education for four years. Since he left work five years ago, his has been a classic retirement in which he has the health, the leisure time to enjoy his retirement, and a retirement income of about $30,600 a year from pensions, investments and Social Security.

Even for Holsinger and his wife, Dorothea, who are better off than most retired teachers, the key to their comfortable retirement, they say, has been the Social Security income and the automatic annual increases.

Currently, Francis Holsinger, 68, receives a Social Security check for $520 and a monthly pension check of $1,350. Dorothea Holsinger, 62, collects another $250 in Social Security and a $100 monthly pension. Investments bring in another $4,000 a year. The Holsingers will receive an additional $86 when their Social Security checks come in tomorrow.

"I'd say we're in pretty good shape," said Holsinger, who spends the summer months in his oceanside home in Ocean City and winters at the family residence in Denton, Md. "But with travel, the two cars, the boat and the two houses to maintain, we still have a lot of expenses. The Social Security increases have meant we haven't had to lower our standard of living since we retired."