Legal tax breaks have allowed corporations and individuals to escape $322 billion in taxes in fiscal 1984, according to a study by the congressional Joint Committee on Taxation.

The study shows that because of so-called tax expenditures -- exclusions, credits, deductions, preferential tax rates and deferrals of tax liability -- the Treasury would receive $513.8 billion less than without the tax breaks by 1989.

Most of the tax breaks go to individuals, the report said. During the government's recently ended fiscal year 1984, tax expenditures of individuals were $246.8 billion, while those of corporations were $75.2 billion. By 1989, the figures are expected to rise to $397.8 billion for individuals and $116.0 billion for corporations.

One of the initial purposes stated in the report was to provide Congress with information necessary to select "between a tax or an outlay approach to accomplish a given goal of public policy." The information also could be used in next year's tax reform debate, which will examine plans to eliminate or restrict many deductions or exemptions.

The largest tax expenditure was for the deductibility of mortgage interest on owner-occupied homes, which was $23.5 billion in 1984 and should rise to $39.8 billion in 1989. Other expenditures estimated:

* Deductibility of property tax on owner-occupied homes, $8.8 billion in 1984; $15.4 billion in 1989.

* Deferral of capital gains on home sales, $4.9 billion in 1984; $7.5 billion in 1989.

* Accelerated depreciation on equipment other than leased property for corporations, $10.2 billion in 1984; $14.1 billion in 1989.

* Investment credit other than employe stock ownership, structure rehabilitation, reforestation, leasing and energy property, $24 billion in 1984; $41.3 billion in 1989.

* Exclusion of employer contributions for medical premiums and medical care for individuals, $17.6 billion in 1984; $34.1 billion in 1989. Net exclusion of pension contributions and earnings in employer plans, $47.2 billion in 1984; $87.9 billion in 1989.