Giant Food Inc. has surpassed Safeway Stores to become the biggest food retailer in the Washington area, according to a new study of the local supermarket business.

With more than $750 million of sales a year in the Washington area, Giant has captured 31 percent of the market, according to estimates of Food World, a regional food merchandising newspaper.

Safeway's share of the market has slipped to 26.7 percent, with sales estimated at $650 million a year, the study said.

Actual local sales of supermarkets are a closely guarded trade secrets, but previous estimates of the Washington area have shown Giant and Safeway neck to neck. One earlier and widely quoted study gave Safeway 33 percent of the market and Giant 30 percent.

Jeff Metzger, publisher of Food World, said yesterday that his study, which is being widely discussed in the supermarket business, is based on six months of data from "sources I consider unimpeachable."

"If you worked day to day in the trade it would be no secret that Giant has overtaken Safeway in the market," he said. "Giant's clout has increased in the last year."

"I don't know where he got the numbers and I can't comment on them," said Safeway spokesman Ernie Moore. Giant press representative Barry Scher could not be reached for comment.

Other supermarket industry sources suggested the Food World estimate of Safeway's share of the market might be on the low side, but agreed that Giant probably has a bigger market share.

In the District of Columbia, however, Safeway is ranked first by the Food World study with estimated sales of $176 million and 38 percent of the market compared with $54 million for Giant and about 12 percent of the market.

Safeway has 33 stores in the District, compared with 7 for Giant. Giant has 85 stores and Safeway 134 in the greater Washington area, defined in the study as including Montgomery, Prince George's and Charles counties in Maryland and Arlington, Fairfax, Loudoun and Prince William counties in Virginia, plus the cities of Alexandria, Falls Church, Fairfax and Washington.

Food sales for the area are estimated at $2.4 billion a year by Food World, which is published in Columbia, Md., and circulates to food retailers, wholesalers and distributers.

After Giant and Safeway, the study ranks the Great Atlantic & Pacific Tea Co. third with a 9 percent share of market, followed by Grand Union with 6.4 percent and Memco (a unit of Lucky Stores) with about 2 percent.

Ranked sixth in the market and first among independents are the three privately owned Magruder's stores on Connecticut Avenue in Washington and in Bethesda and Vienna.

Magruders has grown into a nearly $35 million a year business and its $11.6 million sales per store is the highest average in the market, the study estimates.

Giant's super markets average $8.9 million per store and the smaller Safeway stores average $4.8 million, Food World said.

Both Giant and Safeway are attempting to increase their share of the local market, using different tactics.

The Food World study did not include sales of government commissaries and post exchange stores, which are estimated to account for up to 8 percent of local volume. In the study, Food Fair, Jumbo, Acme and Consumers Co-op rounded out the top 10.

Giant has been pouring millions of dollars into its computerized checkouts and by mid-August expects to have them in all stores - becoming the first major supermarket chain in the nation to eliminate punching of prices into a cash register.

Giant officials say the computer checkouts bring more business to its stores by speeding checkout lines and eliminating errors.

Safeway began last year a $22 million expansion program in the Washington area that calls for enlarging some stores and replacing others with bigger units.

The Oakland, Calif-based firm is building a new 44,000-square-foot store behind its Georgetown unit and will tear the old 27,000-square-foot store down next year. The same thing is planned for the Safeway at Wisconsin Avenue and Ellicott Street NW.

Later this month, Safeway will open a new store at Burke Town Shopping Center in Fairfax County, to be followed in August by a store near Fairfax Circle. Also to be expanded are Safeway units at Hillcrest Heights, Upper Marlboro and Clinton, Md. and in Alexandria and Burke, Va.

Both chains also are expanding in inner-city Washington. Safeway plans an addition to its store at 14th and Corcoran Streets NW and Giant is building a supermarket at the nearby O Street Market site.