On the surface, the tax plan proposed by Senate Finance Committee Chairman Bob Packwood (R-Ore.) appears to offer reductions in individuals' taxes almost exactly the size of those they would receive under the tax-revision legislation passed by the House late last year.

The average tax cut for individuals in the Packwood proposal, which the committee takes up today, would be 8.4 percent, with larger cuts for low-income taxpayers. The House bill would cut taxes on average by 9.1 percent, also cutting more at the bottom end. At any given level of income, the tax cuts are roughly the same size in both plans.

But those averages conceal two new wrinkles in the Packwood plan that could make its effect on individual taxpayers quite different from the effects of the House measure. Lower-income taxpayers, while apt to be happy with their large tax cut, might find they face higher consumer prices; wealthier taxpayers, while pleased with their lower tax rate, could find they face cutbacks in tax deductions they used to enjoy.

The most controversial new element of the Packwood plan would end the federal tax deduction companies now take for excise taxes and tariffs they pay, a change many economists predict will lead to price increases. For taxpayers with low incomes, higher prices could offset some or all of their income-tax cut, economists contend. And the poorest, who pay no taxes, could face higher prices with no offset.

"I really think there is a truth-in-packaging problem here," said Harvey Galper, a tax economist with the Brookings Institution. "It is indeed an increase in sales taxation."

The second innovation in Packwood's plan would limit the increase in the personal exemption for wealthy individuals. Taxpayers with incomes up to $100,000 would get the full increase to $2,000 of the personal exemption. The exemption would phase down to zero for those with incomes greater than $200,000. While that limit slants the direction of the tax cut away from the wealthy, it also raises questions about whether Packwood has really managed to keep the top individual tax rate at 35 percent, as President Reagan requested.

The plan would remove 6.5 million people from the tax rolls, and taxpayers with income between $10,000 and $20,000 would get a tax reduction of 23 percent. Taxpayers making up to $40,000 would get tax cuts of over 9 percent, and taxpayers with incomes greater than $40,000 would have their taxes reduced by a minimum of 4.2 percent and a maximum of 7.3 percent.

According to calculations made for The Washington Post by the accounting firm of Touche Ross, a single taxpayer with a salary of $25,000 and average deductions would pay $3,500 in taxes under the Packwood plan, a 2 percent decline from the $3,573 paid now; under the House bill, the taxpayer would have a slight tax increase, to $3,586.

In a sharp divergence from the House bill, a married taxpayer with a nonearning spouse and two children, making $40,000, would get a 16.6 percent cut under the Packwood proposal, compared to the 33 percent reduction the House bill would provide. A family with two working parents, two children and $65,000 in income would pay 7.3 percent less under Packwood and 6 percent less under the House bill. And a two-child family with two earners making $120,000 would pay 2.2 percent less under Packwood and 7.3 percent less under the House bill.

The deductions used by Touche Ross in making these calculations are based on average deductions for each income level.

The firm's calculations also revealed another possible consequence of the Packwood plan: Higher-income taxpayers face more complexity in filing their taxes, an ironic result for a crusade that began in the name of tax simplification.

"I found it offensively complicated," said Gillian Spooner, a Touche Ross accountant.

Not included in any of the figures produced by the committee or the accounting firm are the indirect effects of ending the excise-tax and tariff deduction. The excise-tax deduction affects mostly alcohol, tobacco and gasoline; the end to the tariff deduction could raise the prices of imported goods such as clothing. The change is expected to raise $62 billion in federal revenue over five years.

Because no one can calculate the extent to which corporations would pass the costs along or exactly which taxpayers would pay the higher costs, the "distributional effects" of that change are not included in the official estimates.

Packwood aides say there is no economic difference between ending the deduction for excise taxes and doing away with other breaks in the corporate income tax. Packwood has also decried the unfairness of the current system, in which, he says, corporations collect the full excise tax amount from consumers and then soften the impact on themselves by deducting it as a cost of doing business.

Joseph Minarik of the Urban Institute responded that economists have generally concluded that the burden of the corporate income tax falls on stockholders and other owners of capital. And he notes that companies must report their excise tax collections as income, canceling out the tax advantage of the deduction.

Sen. George J. Mitchell (D-Maine), a member of the Finance Committee, said yesterday he will ask Packwood at today's meeting to hold hearings on the excise-tax proposal, and said he has asked the Joint Committee on Taxation to calculate how individuals would be indirectly affected.

Sen. Bill Bradley (D-N.J.) agreed that there would be pressure to change the excise-tax provision if it were found to lead to higher consumer prices. He suggested that as an alternative, the committee could drop a Packwood proposal to let companies "sell" their unused investment tax credits back to the government.