When the U.S. Census Bureau measures living arrangements, "households" and "housing units" relate numerically as follows: In 100-percent tabulations the count of households always equals the count of occupied housing units; but in sample tabulations, the numbers may differ as a result of the weighting process.
The Census defines the following:
A household includes all the persons who occupy a housing unit. The occupants may be a single family, one person living alone, two or more families living together, or any other group of related or unrelated persons who share living arrangements.
A housing unit is a house, an apartment, a mobile home, a group of rooms, or a single room that is occupied (or if vacant, is intended for occupancy) as separate living quarters.
Separate living quarters are those in which the occupants live and eat separately from any other persons in the building and which have direct access from the outside of the building or through a common hall.
People not living in households are classified as living in group quarters.