Retail sales in the Washington metropolitan area rose about 12.5 percent last year to a record $10.8 billion, according to figures compiled from Commerce Department reports.

In December alone, sales boomed beyond all forecasts to $1.1 billion -- 22 percent over the previous month and about 12 percent ahead of the same month in 1976.

The overall area sales growth outpaced a 10 percent national average increase and a gain of 11 percent in the South Atlantic states, which include Maryland, Virginia and the District.

Area consumer prices rose 7.2 in 1977, indicating a real increase in merchandise sales of about 5 percent. Washington area sales in 1976 had increased a modest 8 percent over 1975 --most of which represented inflated prices.

Despite the overall metropolitan sales rebound, department stores in downtown Washington continued to suffer a slight decline in business.

For the year as a whole, sales at the three major downtown stores declined more than 1 percent to about $92 million, following a 2 percent decline in 1976. In December alone, however, downtown sales were up slightly from the same month in 1976.

Areawide department store sales rose about 8 percent last year to an estimated $1.4 billion. December department store sales of $231 million were 65 percent higher than the $140 million volume of the previous month.

Figures for 1977 were compiled from Commerce Department estimates for recent months, based on a revised formula for gathering sales data, plus earlier information published by the agency under the old formula.