The Soviet Union is considerably less self-sufficient and more dependent on imports than previously believed, the Census Bureau said in a study yesterday.

Census director Bruce Chapman said, "the implication . . . is that the Soviet Union is also far more vulnerable to economic sanctions" than previously thought. Use of such sanctions, to retaliate for such actions as the imposition of martial law in Poland, has been a continuing issue in the Reagan administration.

In all, the new study suggests the Soviet Union is about as dependent on international trade as is the United States.

The study shows that Russian imports in 1980 amounted to 20 percent of Soviet national income. That figure is two to three times higher than previously estimated. The reason is that the study's authors, for the first time, were able to estimate imports based on the real value of the Russian ruble in the Soviet economy, derived from domestic prices.

The lower previous figures were based on the formal exchange rate for the ruble, which is not used in international trade, and they overstated its value. In effect, the previous figures understated the value of Soviet imports and overstated the value of their exports. The new study put the value of Soviet exports at about 7 percent of Soviet national income.

While the study, which was done by the Census Bureau's Barry L. Kostinsky and Duke University professor Vladimir G. Treml, appeared to support use of economic sanctions against the Soviets, as advocated by many hardliners, Chapman said at a news conference it was not undertaken for that purpose but was one of many assessments regularly made by the bureau's foreign demographic analysis division.

Those advocating economic sanctions--for example, prohibitions against sale to the Soviets of U.S. machinery needed to complete the proposed natural gas pipeline from Siberia to western Europe--say not only that the Soviets depend to a large extent on international trade, but also that their economy is in bad shape, and vulnerable on that count as well.

Kostinsky said that 15 to 20 percent of all machinery and equipment put into use in the Soviet Union each year is imported. For the chemical industry this figure is perhaps 50 percent, and among advanced computer-programmable machine tools 50 to 80 percent have major, vital foreign-made components.

The Soviets also depend heavily on imported grains for food and animal feed, Treml said. The Soviets produce more than 100 million metric tons of grains a year and imports have equaled 10 million to 20 million tons routinely in recent years and there are some rumors this could reach 40 million tons, he said.

Treml and Kostinsky said that for many years western scholars had believed that the Soviet Union was largely self-sufficient, obtaining only 3 to 5 percent of its national income from exports, and importing foreign goods worth about 3 to 5 percent of national income.

However, they said, while these estimates may have been true for the 1930s and 1940s, they have become increasingly misleading in recent years.

In fact, the Soviet Union has sharply increased its dependence on foreign trade, but this fact had been masked by the inability of non-communist scholars to obtain statistics based on the real value of the ruble in the Soviet economy. The new estimates are based on domestic prices as calculated by Treml and Kostinsky and later corroborated by figures in a Soviet publication.

From this they derived these figures:

* Soviet imports valued at internal ruble prices rose from about 12 billion rubles in 1960 (about 8 percent of national income) to about 25 billion in 1970 (9 percent of national income) and 92 billion rubles in 1980 (20 percent of national income). Under the old method of calculating, the 1980 value of imports would have been only about 45 billion rubles and the percent of national income below 10 percent. National income, as used in the Soviet Union, is similar to gross national product in the United States.

* Soviet exports (excluding gold), valued at internal ruble prices, totaled about 5 billion rubles in 1960 (about 3.5 percent of national income), about 10 billion in 1970 and about 32 billion rubles (7 percent of national income) in 1980. Under the old method, the value of exports would have appeared higher.

Using the new method, Kostinsky said, probably a bit over half of all Soviet imports come from non-communist countries and the rest from Soviet bloc nations. Major imports in 1981, he said, were machinery and equipment (30 percent of imports), food and feed grains (28 percent), industrial consumer goods (13 percent), metals and ores (10.3 percent) and chemicals (5 percent).