At least 80 percent of the nation's 86.5 million taxpayers will face a tax increase next Jan. 1, despite the $187 billion tax-cut bill that Congress passed last Sunday.

The increase will hit virtually all taxpayers except those in one-earner families with incomes of less than $19,000 a year. Depending on earnings, it will vary from $4 to $1,826.

The reason for the boost is that the income-tax cuts Congress enacted Sunday won't be enough to offset the higher Social Security taxes scheduled for January 1, or the effect of inflation in pushing workers into higher tax brackets.

Although congressional leaders are correct in arguing that the bill's $13.3 billion in tax cuts for individuals would offset the expected $3.2 billion in higher payroll taxes and $8.8 billion in inflation, that's only in aggregate.

Sen. Russell B. Long (D-La) chairman of the Senate Finance Committee, took pains to point out this past weekend that the tax bill would protect "most Americans" from the Social Secuirty tax increase and one year's inflation.

However, the way the lawmakers fashioned the bill, this protection would be concentrated almost entirely among some of 12 million taxpayers who are part of four-person families earning $19,000 or less.

MoreoverM as Long conceded openly, the protection such as it is covers only one year's inflation. If the impact of all the inflation since the last big tax cut, in 1977, were counted, no one would come out ahead.

The reason for the shortfall essentially is one of money: The Senate wanted to provide $2.1 billion in relief from capital gains taxes. and there was no money left after that to provide breaks for ordinary taxpayers.

Relief from capital gain taxes primarily affects taxpayers in higher income brackets. For example, 68.2 percent of this bill's capital gains breaks go to those earning $50,000 or more. Almost half goes to $100,000 and over.

Figures published by the Treasury show that the 63.6 percent of American taxpayers who earn $15,000 a year or less would receive only 21 per cent of the relief, while the 4 per cent making $100,000 and up would get 12.4 per cent.

Moreover, the reductions would be roughly proportional with most categories of taxpayers receiving about the same percentage-cut - between 6.5 and 7 per cent of their present tax liability.

The major exceptions would be those earning $200,000 and over, who would receive an average 7.6 per cent cut, and those in the $5,000 to $10,000 brackets, who would get a 19.1 per cent cut.

In terms of the share of the total tax-cut pie that Congress dished out last Sunday, the group that came out worst was that in the $10,00 to $15,000 income bracket, which got 7.9 per cent on the relief.

That was because lower-income taxpayers were protected to some extent by the increase Congreee voted in the "earned income credit" for the working poor.But taxpayers earning $10,000 or more do not qualify for the break.