Census questionnaires will be arriving by mail at more than 260,000 District households this week.

The forms, expected to be completed and returned by April 1, will be instrumental in future planning for the District, Jean Judd, manager of the city's census office, explained. She said the census, taken every 10 years, determines the amount of federal funds and services to be allocated the District and other jurisdictions.

"The census is valuable to all citizens," she said, "because it is used to determine the development of shipping centers, roads, schools, the allocation of funding and representation in Congress."

In 1970, there was a 7.9 percent undercount in the District, which subsequently contributed to the city's current budget troubles, Judd said, adding, "Mayor Barry would not be in the rough financial shape he is in now if the 1970 census had been accurately counted."

More than 1,500 people will help with the District head count of city residents. Judd said radio announcements, newspaper stories and leaflets have been used to get the word out.

Special efforts have been made to help residents who do not speak English complete their forms. Every District office will have an aide who can speak Spanish, to assist Latino residents.

Residents who do not complete their forms and return them by April 15 can expect calls or visits from census workers, she said.

According to Judd, the census is completely confidential. She said the information can not be used by any federal or state agency other than the Bureau of the Census. Anyone releasing such information to individuals outside the census agency faces a $5,000 fine or up to five years in jail, she said.

Two types of forms will be sent to District residents. The short form has 19 questions and takes approximately 15 minutes to complete. The longer form has 64 questions and will take about 45 minutes to complete.

One of every six residents will be randomly selected to receive the long form. Residents who require help in filling out the forms may call 737-2300. The number is also supplied on the form.