WASHINGTON (AP) _ Half the jail inmates convicted of drunken driving had consumed the equivalent of at least 12 bottles of beer or eight mixed drinks before their arrest, the federal government said Sunday.

The Bureau of Justice Statistics also reported that almost half of the people jailed on drunken driving charges or serving a sentence for that offense had been sentenced for the same offense previously.

The study found that one in five of the convicted drunken drivers had been consuming two or more kinds of alcoholic beverages when arrested. The study was based on a new analysis of inmates being held in 1983, the peak year for drunken driving arrests, in 407 local jails.

The median age for those in jail for drunken driving was 32, about 5 years older than the median age for all jail inmates. Some 95 percent of the jailed drunken drivers were men and nearly 80 percent were unmarried, separated, divorced or were widows or widowers.

Another portion of the study found that arrests for driving under the influence of alcohol rose more than 220 percent from 1970 to 1986, while the number of licensed drivers increased by just 42 percent in that time.

In 1986, some 1.8 million people were arrested for driving under the influence of alcohol. In 1983 there were some 1.9 million such arrests, one for every 80 drivers.

The study also found that rates for driving under the influence were highest among 21-year-olds, reaching a peak in 1983 with a rate of one arrest for every 39 licensed drivers of that age.

Since 1983, most states have phased in new laws raising the minimum age for buying or selling alcohol to 21. Drunken driving rates for those ages 18 to 20 have declined by 14 percent since then, more than twice the rate of decrease for those age 21-24.