Washington — Most Americans' personal financial situation in 1985 will be affected by inflation figures that were released Wednesday. The government reported that the consumer price index (CPI) for September rose a seasonally adjusted 0.4 percent and that, for the first nine months of 1984, prices rose a seasonally adjusted 4.2 percent.
Last month's CPI was the last piece of information the government needed to plug into various formulas that adjust the social security system and the federal income tax system for inflation during the preceding fiscal year, which ended Sept. 30.
As a result, next year social security checks will increase 3.5 percent and social security taxes will rise. At the same time, federal income tax exemptions , deductions, and tax brackets will be adjusted to offset the impact of inflation. Moreover, personal income tax withholding rates will be lowered in January, when indexing is scheduled to begin. So individuals whose salaries in 1985 do not keep pace with inflation will enjoy a reduction in federal income taxes.
September's inflation data also provide some reassurance for those forecasters who are predicting moderate inflation in 1985.
''Prices continue to be moderate,'' says David Wyss, senior vice-president at Data Resources Inc., a forecasting firm. The firm expects prices to rise less than 4 percent next year, vs. 4.25 percent for all of 1984.
Changes that the latest price figures trigger include:
* A rise in social security benefits for retirees, widows, and the disabled, starting with checks delivered Jan. 3. The average retired couple will see its check jump $26, to $776. The average retired widow will see her check climb $14, to $415. Benefits will also be adjusted for people receiving supplemental security income (SSI), a federal welfare program that helps the needy, aged, blind, and disabled. A single person on SSI will now get $11 more, or $325 per month, starting with the Dec. 31 check.
* An increase in the level of wages subject to social security taxes. The maximum amount will climb $1,800, to $39,600, on Jan. 1. The social security tax rate that workers pay on their wages was already scheduled to rise from an effective 6.7 percent this year to 7.05 percent in 1985.
The social security tax rate paid by employers will rise from 7.0 this year to 7.05 in 1985. Workers and employers normally pay an equal tax rate, but in 1984 workers received a tax credit that reduced their tax rate.
As a result of the higher taxable wage base and the higher tax rate, the maximum social security tax next year will rise $259.20, to $2,791.80, according to the Social Security Administration.
* A boost in the $1,000 personal and dependency exemptions, as well as in the standard deduction, used to figure federal income taxes. Tax brackets also will be widened so inflation does not cause ''bracket creep.'' That occurs when a worker's wage rises to keep pace with inflation, pushing him into a higher tax bracket and boosting his tax bill.
For the year ending in September, the adjustment factor for exemptions and tax brackets was 4.08 percent, the Treasury Department said. That means that a taxpayer can now claim $1,040 for each exemption.
Each tax bracket also will be boosted by 4.08 percent. For example, the bottom tax bracket for married couples currently ranges from $3,400 to $5,400 and taxes the last dollar earned at an 11 percent rate. After indexing, the 11 percent tax bracket now will stretch from about $3,539 to $5,620.
To account for the wider brackets, withholding rates also will be lowered 4. 08 percent.
Those individuals whose 1985 pay climbs by less than that will enjoy a tax cut. New withholding tables will be released by Dec. 15, an Internal Revenue Service spokesman said.
The impact of indexing is made clear by estimates made by the Associated Press. The AP figures that indexing will save a couple with one wage earner $38 if income is $20,000, $85 if income is $30,000, $124 if income is $40,000, and $ 420 if income is $100,000.
The higher social security benefits will cost about $6.1 billion, which will be offset by higher social security taxes.
Indexing of the tax system for inflation will cut federal revenues an estimated $5.4 billion in fiscal year 1985, the Congressional Budget Office says. The revenue reduction rises to $45.4 billion by 1988.
Social security benefits are increased when the average consumer price level (measured by the CPI for urban wage earners and all clerical workers) for the July-to-September period is 3 percent or more above the average level of the same quarter in the previous year. President Reagan had proposed a waiver of the 3 percent trigger for 1984 in case inflation for that period did not equal 3 percent.
When benefits are boosted, the taxable wage base and the amount retirees can earn without sacrificing benefits also increase to reflect changes in the average wages covered under the social security system.
Beneficiaries under 65 will be able to earn $5,400 in 1985, vs. $5,160 now, without losing benefits.
Those 65 to 69 will be able to earn $7,320 in 1985 vs. $6,960 now. The amount of earnings required to earn a quarter of coverage under the system will increase $20 in 1985, to $410.