Americans have sheltered about $20 billion this year in tax-deferred retirement plans, and about 60 percent of that money might not otherwise be saved, according to estimates by government regulators and industrial trade associations.

Consequently, contributions to Individual Retirement Accounts during the first taxable year in which they are open to all workers may reach $35 billion.

During the debate preceding creation of the universal IRA, the most conservative forecasts, based on a previous participation rate of just 5 percent of those eligible, were in the $5-billion-to-$10-billion range.

The most optimistic guestimates cited a potential of $50 billion, presuming all eligible investors made contributions. Observers attribute the better-than-anticipated response primarily to the barrage of publicity that has surrounded IRAs but also to the tendency to save during a recession.

Figures released in mid-July by the Federal Reserve, together with earlier estimates by the U.S. League of Savings Associations, the Credit Union National Administration and the Investment Company Institute show contributions of $18 billion. (The numbers cover those previously eligible as well as those newly eligible.) However, some figures cover only the first quarter and others include Keogh contributions as well. Also, IRAs set up through insurance companies, brokers and employers are not included.

A survey of consumers conducted by the Life Insurance Marketing and Research Association of Hartford tends to corroborate the responses of financial institutions. LIMRA polled 3,634 households during April and found that 17 percent of the 86 million U.S. households that were both eligible for and aware of IRAs had already opened accounts. The average contribution per household amounted to $1,835, making a total of $17.9 billion.

In general most of the money has come from middle-aged persons in the upper brackets. Half of those aged 45 to 64, with a household income of $30,000 or more, have bought IRAs. New England leads the nation with 24 percent of those eligible already signed up; the middle Atlantic states were next with 21 percent.

Another 16 percent definitely plan to purchase, while 32 percent said they were undecided. Of the 35 percent who do not intend to buy an IRA, most said they couldn't afford it. The law allows a maximum annual contribution of $2,000 per worker ($2,250 by a worker with a nonworking spouse), but sets no minimum.

Based on patterns to date, LIMRA projects sales of up to $35 billion by April 15, 1983, the last date for contributions for the tax year 1982. The calculation is based on the assumption first that the number of persons who said they planned to buy but don't will cancel out the number of those now undecided who do opt to contribute.

Second, the 16 percent planning to buy will put in less money than the wealthier consumers who jumped in right away.

On the other hand, there may well be a surge of contributions in early April 1983 just as there was this year. This year, for example, the Federal Reserve noted that sales of ceiling-free IRA/Keogh deposits at commercial banks alone shot up from $3.9 billion in the first three months to $6.5 billion by the end of April.

As for share of the market, LIMRA's study shows that commercial and savings banks have captured about two-thirds of the IRA deposits. The percentages are 32 percent at commercial banks, and 34 percent at thrift institutions. This compares with the 42-28 percent split found by an American Bankers Association study in March.

According to Federal Reserve and U.S. League data, commercial banks have now captured $7.3 billion in new deposits, while savings and loans have $7.1 billion and savings banks, $1 billion.

Credit unions drew in $301 million in the first month, but no update has been done. LIMRA gives credit unions a 7 percent share of the market, which would put contributions over $1 billion. However, that figure may be smaller because the average contribution from credit union members is smaller, about $1,385, according to the Credit Union National Association.

At one time, the mutual fund industry said it aimed at capturing a third of all IRA contributions. LIMRA's survey put the market share at just 6 percent, or $1.2 billion. This matches the industry's estimate for the first quarter. The Investment Company Institute says 70 percent of IRA money went into money market funds.

Stockbrokerage firms were given a 9 percent share, or $1.6 billion. Eighty-seven percent of the households using a broker put in the maximum amount allowable. The study noted that the use of brokers increased with level of income while the use of banks decreased, until age 55. Just 2 percent of the consumers questioned made payroll contributions to IRAs.

In the survey, life insurance companies claimed a 16 percent market share, or $2.8 billion, considerably more than the 10 percent the industry had predicted. Prudential, for example, took in $139 million during the first quarter.