Starting Jan. 1, Americans' financial relationship with Uncle Sam will change in a number of significant ways. Several of the shifts affect individuals' federal taxes.
* In 1985, for the first time, the tax system will be indexed. So salary hikes that just offset inflation do not boost an individual's tax bill.
The personal exemption and standard deduction will be increased by 4.08 percent and tax brackets widened the same amount to neutralize the effect of inflation in 1984 on the tax system. The result is that ''most taxpayers will see a decrease in withholding'' from their paychecks in 1985, says Internal Revenue Service spokesman Wilson Fadely.
* The 1984 tax forms now arriving in mailbox es around the country reflect this year's revisions in the tax code. Rules taking effect on the 1984 forms change the way that charitable contributions, social security benefits, and capital gains on stock, among other things, are taxed.
The New Year also will bring significant changes in social security benefits and taxes.
* To offset inflation in 1984, social security checks due to be delivered Jan. 3 will be 3.5 percent higher than those received in December. The average retired couple will see its check climb $26 to $776. The average retired widow will see her check rise $14 to $415.
* To help pay for bigger benefit checks, the portion of a worker's wages subject to social security tax and the rate at which that income is taxed will be boosted.
The maximum amount subject to social security tax will climb $1,800 to $39, 600. The tax rate workers pay will jump from 6.7 percent in 1984 to 7.05 in '85.
As a result of the higher taxable wage base and the higher tax rate, the maximum social security tax payment next year will rise $259.20 to $2,791.80, according to the Social Security Administration.
The effect of these changes will vary with an individual's or family's circumstances.
For many moderate- and low-income taxpayers, the benefits of lower federal income tax withholding will be offset by higher social security taxes.
For example, a family of four with one wage earner who brings home $25,000 will pay $51 less in federal taxes in 1985 due to indexation, according to calculations made by the Associated Press. But the family's wage earner will pay in financial circumstances, Uncle Sam will collect $37 more in withholding for income and social security taxes from that family in 1985 than in '84.
At the upper end of the income scale, benefits from indexing are more likely to offset higher social security taxes. That is because social security tax is levied only on the first $39,600 of income. So a four-member family with one wage earner who brings home $100,000 will save $420 in 1985 as a result of indexation, the AP calculates. But the wage earner would pay only $259.20 more in social security taxes. So they will send $160 less in withholding for income and social security taxes to Washington in 1985 than the year before.
Not every worker will find his first paycheck in 1985 fatter on acount of tax changes. ''There may be a few whose withholding stays the same,'' IRS spokesman Fadely cautions. That is the result of technical changes the Treasury Department made in calculating the 1985 withholding tables used by employers.
Different formulas are used to adjust the social security and federal tax systems for inflation. But both formulas measure the change in inflation during the preceeding federal budget year, which ends Sept. 30.
Adjusting the income-tax system for inflation means that in 1985 the personal exemption will be worth $1,040 rather than $1,000. The standard deduction for a single taxpayer will be worth $2,390 rather than $2,300. On a joint return the standard deduction will be worth $3,540 rather than $3,400.
Under rules taking effect on the 1984 tax forms, individuals who do not itemize deductions can deduct up to 25 percent of the first $300 of their charitable contributions, up to a limit of $75. The old rule limited such deductions to no more than 25 percent of the first $100 in contributions, up to a limit of $25.
In addition, up to half of social security benefts will be taxable if total income exceeds $25,000 for a single person or $32,000 for a couple filing jointly.
And on stock or other capital assets acquired after June 22, 1984, the required holding period to obtain favorable tax treatment is reduced to six months. For assets acquired prior to that date, the holding period remains one year.