Federal estate taxes on the wealth a person leaves to a spouse, children or other beneficiaries at death will be eliminated for all but a small fraction of the largest estates when the new tax law becomes fully effective in 1987.

A husband and wife who took full advantage of the new rules for gifts and bequests could leave $450,000 tax-free to beneficiaries next year, for example. And by 1987, after the tax act has been phased in, the total will rise to $1.2 million, tax experts say.

"Many fairly large estates can probably be brought into the tax-free range by an effective program of lifetime gifts and other planning techniques," according to an analysis of the 1981 Tax Act by Touche Ross and Co.

Under current law, some 2.8 percent of estates are subject to federal estate taxes, but when the new law takes effect on Jan. 1, that number is expected to shrink to less than one-half of one percent.

In addition, the tax rates on the largest estates above the tax-free range have been substantially reduced. The current maximum rate of 70 percent on estates of $5 million or more will drop in steps to a top rate of 50 percent on estates of $2.5 million or more beginning in 1985.

The principal changes are these, although special rules will affect the impact of the new tax on the largest estates:

Old law: Taxpayers have been eligible for a tax credit to offset taxable gifts made while living, with the balance available to reduce taxes on estates. Through this year, the unified gift-estate tax credit totals $47,000. Since this is a credit, directly reducing the final tax obligation rather than a deduction that cuts a person's taxable income, the effect is substantially larger: The unified credit completely exempts gift and estate transfers up to $175,625.

New law: Beginning next year, the credit is increased to $62,800, exempting estate and gift transfers up to $225,000. The credit grows in stages until it reaches $192,800 in 1987. This will eliminate estate taxes on estate or gift transfers of up to $600,000.

Old law: A taxpayer can leave $250,000 or one-half of an estate, whichever is greater, to a spouse tax-free. In addition, there is no tax on the first $100,000 of gifts from one spouse to another.

New law: An unlimited estate tax marital deduction is provided for those who die on or after Jan. 1, 1982, along with a 100 percent marital deduction for gifts to spouses made on or after that date.

Also, the amount of tax-free individual gifts has been increased from the current limit of $3,000 per recipient annually to $10,000 in 1982. A husband and wife could give $20,000 tax free a year to each of their children, or any other individual.

Tax consultants caution that because of the much larger value of the unified gift and estate tax credit, both spouses should be certain to take full advantage of the gift provisions to obtain the maximum tax savings.

Old law: Gifts made within three years of death have been included within the gross estate and valued as of date of death, in part as an attempt to prevent a taxpayer from reducing estate taxes by making gifts at a time death is imminent.

New law: The rule is reversed. Beginning next year, most gifts made within three years of death won't be included in the estate tax determinations, although special rules govern certain gifts, such as life insurance benefits.

Old law: Farms and small businesses are permitted to be assessed not at the market value based on the "highest and best use," as other assets would be, but instead on their current value. The "highest" use of a large farm near a rapidly expanding urban area, for instance, might be as a new housing development. Such a determination would multiply the estate tax on such a property, making it very hard for children to continue working a farm left by a parent. Congress enacted this provision with the hope of maintaining family farms.

It included a limit, however, that the reduction value based on current use couldn't exceed $500,000.

New law: That limit is increased to $600,000 this year, $700,000 in 1982 and $750,000 thereafter.