Taxpayer Ronald Reagan saved $91,619 on his 1982 taxes--about one-third of what he otherwise would have owed--as a result of legislation signed in the last two years by President Ronald Reagan.

But he owed his good fortune in part to Congress as well as to his own proposals. Congress sweetened his recommendations in a way that gave him a bigger tax cut for 1982 than he would have given himself.

The president and Mrs. Reagan, who filed their tax return April 14, would have paid $384,235 in taxes without the benefit of the new legislation. But with the amendments to the code, principally the cut in the maximum tax on unearned income from 70 to 50 percent, they paid $292,616.

The savings were confirmed in an analysis of the Reagans' tax returns by Ross L. Collins II, a certified public accountant in McLean, at the request of The Washington Post.

The tax on the Reagans' 1982 income of $741,253 was calculated by Ross on both a 1980 tax form--the last in use before his proposals took effect--and a 1982 tax form, keeping the amounts and types of income the same. The bottom-line difference was that changes in the law saved the Reagans $91,619--more than 10 percent of their adjusted gross income and nearly a third of the total taxes they otherwise would have owed.

Reagan has been criticized for pushing tax cuts mainly for the rich while cutting social programs meant to protect the poor. His own return is an illustration of what his policies meant to one rich family. The Reagans still paid almost 40 percent of their income in taxes, however.

Not all of the Reagans' tax savings can be directly linked to the president's original "supply-side" plan to reinvigorate the economy by cutting taxes and putting more money into the hands of investors and buyers.

Reagan had planned to reduce the maximum tax on unearned or investment income--dividends, interest, capital gains from the sale of real estate or other assets--from 70 to 50 percent gradually over three years. But Congress offered, and Reagan accepted, an immediate cut in this tax rate to 50 percent in 1981.

The Reagans' largest single source of income last year was unearned income from the sale of their home in the Pacific Palisades section of Los Angeles for $1,000,100. The first family reported a capital gain of $256,978 on the sale, and that gain was taxed at the lower rate.

They saved another $5,000 as a result of the increase--from $100,000 to $125,000--in the maximum tax-free gain allowed on the sale of a principal residence by persons over age 55.

Collins, the accountant, points out that Reagan could have saved himself even more money had he purchased an All-Savers Certificate; the 1981 bill authorized such certificates, on which limited amounts of interest are tax free.

And there is another point to be made: If history is any guide, Congress would have cut taxes sometime over the last two years with or without Reagan. It might not have cut them as much or the same way as it did. But to offset inflation's effect in lifting people into higher tax brackets it has voted tax cuts every few years in the past, and there is no reason to think it would have stopped now.