Tax bill saves a few hundred dollars for most people. Phase-in of changes makes it hard to pinpoint bill's effects.

``Everybody on average will pay less. Of course, nobody's average.'' That is the problem in trying to figure out how the new tax bill will affect individuals and families, says Steven Corrick at Arthur Andersen & Co., the accounting firm.

``Sure it lowers the rate,'' Mr. Corrick adds, ``but it expands the tax base in ways even we are just beginning to fathom. It's virtually impossible to tell if you're better or worse off.''

Impossible or not, the very fact that the tax bill was approved by the House-Senate conference committee last weekend has sent accountants and financial planners scurrying to their computers and calculators trying to answer this week's most-often-asked question: ``How will it affect me?''

The answer depends on your income and the way you save on taxes now. The main piece of advice: Read all you can about the tax law. All the newspapers and newsmagazines are full of the information this week. Also, in general, it makes sense to take whatever deductions you can this year, since they may not be around next year.

``All you can do now is get out your 1985 tax return and rework it under this proposal,'' says Steven Enright, director of financial planning at Seidman Financial Services.

The bill will affect every taxpayer in one way or another:

Many people will pay taxes at lower -- or at least different -- rates, thanks to the eventual elimination of all but two tax brackets.

Everyone will get an increasingly larger personal exemption. It climbs from $1,080 this year to $1,900 in 1987, $1,950 in 1988, and $2,000 in 1989. This will be especially helpful to families with children.

The increase in the standard deduction to $5,000 for joint returns, $4,400 for unmarried heads of households, $3,000 for single people, and $2,500 for married individuals will mean that fewer people will qualify for itemizing deductions in the future.

One reason it's so difficult to nail down how the tax bill will affect individuals and families is that many provisions don't take effect immediately but are phased in over two or more years.

The tax brackets, for example, won't automatically drop from 14 to two. Instead, next year will see five brackets that are a blend of the old and the new. As a result, some people will see their tax bills fall in 1987 and rise a bit in 1988, while others will pay more in 1987 and less in 1988. Still others will pay more both years, since they may have been using several deductions that will no longer be available.

And lower-income taxpayers may notice very little change for a reason that has nothing to do with income taxes: social security.

``The tax proposal doesn't do anything about social security taxes,'' says Thomas Ochsenschlager of Grant Thornton International, an accounting firm. ``The social security tax will be the same before and after tax reform.''

For people in lower-income brackets who were paying little tax to begin with and had more deductions, the social security tax will cost a larger proportion of their take-home pay.

Note, too, that when the new plan does go into effect, the simplicity of having two brackets does not mean that you pay either 15 or 28 percent. For every one except the very wealthy, it's a mixture.

Take a husband and wife, filing jointly, with a taxable income of $30,000. Their first $29,750 would be taxed at 15 percent. Only income above that would be taxed at 28 percent. So there is little incentive for the couple to try to shave stated income by $250 to get under the $29,750 level.

Beyond these basics, the tax bill will change the way millions of taxpayers use or calculate their individual retirement accounts (IRAs), their pensions and company-sponsored retirement savings plans, their investments, their tax shelters, their use of credit, and their savings programs for their children's college education.

If all taxpayers got an equal benefit out of the tax bill, they'd be $204 ahead. But they won't. Instead, a retired couple might see taxes rise more than $400 a year by 1988. A single person making $30,000 might save about $2,000. An executive earning more than $300,000 could see taxes slashed by more than $25,000. If this executive had been investing heavily in tax shelters, however, he might be faced with a bigger tax bite.

``It's important to remember that we're talking about a tax savings of no more than a few hundred dollars for most people,'' says Pamela Pecharich of Coopers & Lybrand, another accounting firm.

One accountant sifting through the data from his computer is Joel M. Walters, also of Grant Thornton. Until now, Mr. Walters figures, the elderly and blind received an additional personal exemption, which this year amounts to $1,080. Under the tax bill, they will get an extra standard deduction instead. Single individuals will receive an extra $750 and married people an extra $600. Had they been getting an exemption, they would have saved $2,000 by 1989.

On the other hand, a single parent raising two children could save more than $600 a year. This family benefits greatly from the larger personal exemption, saving it $6,000 by 1989.

While it is difficult to figure out how the tax bill would affect the average taxpayer, it is useful to see how various income groups stand to gain or lose.

The $204 saving represents an average cut of 6.1 percent, according to figures from the Joint Committee on Taxation. For someone earning less than $10,000, the law would save 65.7 percent in taxes by 1988, while those in the $10,000-to-$20,000 range would save an average of 22.3 percent.

Americans earning $20,000 to $30,000 would get a tax cut of 9.8 percent, and those making $30,000 to $40,000 would save 7.7 percent. In the $40,000-to-$50,000 range, the average savings would be 9.1 percent.

Between $50,000 and $75,000, the law would save just 1.7 percent, and the tax cut would be an even 1 percent for those earning between $75,000 and $100,000. People earning $100,000 to $200,000 would see an average tax cut of 2.4 percent; above $200,000, it would be 2.3 percent.

Thus many middle- and upper-middle income families could end up paying more, if they don't have several personal exemptions and have relied on deductions to cut their tax bill.

``The income class between $50,000 and $100,000, or a significant part of that class, will see their taxes increase,'' Ms. Pecharich says. ``Below that, taxes will decrease, and above that, over time, there will also be decreases.''

Not only will tax reform change the rules and rates, it will also dramatically change Americans' financial behavior, notes Mr. Enright of Seidman Financial Services: ``People will have to start thinking through how they handle their purchases.'' The loss of the consumer interest deduction will force more people to save for -- or put off -- some purchases.

``It's sure not going to help the auto industry,'' he says.