About 86 percent of all the federal income tax benefits enacted by Congress last year at President Reagan's request will go to families with incomes over $20,000 a year, while two-thirds of all the cuts in benefits will be taken from families below $20,000, according to the Congressional Budget Office.

The CBO figures were contained in a study requested by Sen. Ernest F. Hollings (D-S.C.) and Rep. James R. Jones (D-Okla.), chairman of the House Budget Committee.

Hollings and Jones, who made the study public yesterday, said caustically that "fairness is a critical element in any budget" and "the CBO study speaks clearly on the fairness of the Reagan program."

The study showed that low-income families would realize very small tax savings from Reagan's tax cuts as enacted, while high-income families would do extremely well.

A family with income under $10,000 would average only $120 a year in tax savings. One with income from $10,000 to $20,000 would average $440; one with income from $20,000 to $40,000 would average $950 in tax savings, and one with income from $40,000 to $80,000 would average $1,830.

But a family with income over $80,000 would average $15,250 in tax reductions. Those families would end up with a fifth of the $82 billion in 1983 tax savings.

In computing reductions of federal welfare and related benefits to different income groups, the CBO took into account cuts voted last year in Social Security, Medicaid, Medicare, unemployment insurance, food stamps, welfare, fuel aid, educational benefits and veterans' programs.

The expected reductions in fiscal 1983 totaled $17.5 billion. About $6.9 billion of that will be cut from families with incomes under $10,000, and $4.6 billion from families with incomes from $10,000 to $20,000, so about two-thirds of the cut comes from families under the $20,000 figure.

The average family under $10,000 will lose $360 in benefits, while gaining only $120 a year from the tax cuts, so it will have a net loss of $240. However, families from $10,000 up will have a net gain from the combined impact of the tax cut and benefit cuts.

The CBO study, looking at federal grants to states and localities, said cuts would range from $12 billion to $14 billion from fiscal 1982 to 1984, and about three-fifths would come from programs targeted toward low-income individuals.

Although most of the benefit cuts come from low-income people, the CBO said the average family under $10,000 would lose only a small portion of its income, about 3 percent, from reductions in cash benefits.

But it said certain types of households would be much more heavily affected. "In the low-income category, for example, households that receive Aid to Families with Dependent Children benefits and food stamps will lose about 5 to 10 percent of their total incomes on average if they have no earnings and up to 20 percent of their incomes if they are unemployed." These calculations tends to confirm other studies that said the "working poor" would be more heavily hit by the 1981 cuts than other groups.