The median income of all U.S. families reached $17,640 last year, a rise of 2.4 percent over 1977 after adjusting for inflation, the Census Bureau reported yesterday.

Median income is the amount halfway between the highest and the lowest. Half of all families earned more than the median, half earned less.

In 1977, the actual dollar median income of U.S. families was $16,009 -- so in nominal dollar terms median income rose about $1,600 in 1978. But after adjusting for dollar loss of purchasing power as a result of inflation the real increase during 1978 in terms of constant 1978 dollars was only about $414 -- 2.4 percent.

The bureau also calculated that in terms of real spending power, median family income rose 6.5 percent from 1970 through 1978, after adjusting for inflation. This is an average of eight-tenths of 1 percent a year -- a far slower pace than from 1960 to 1970, when the average rise after inflation adjustment was 3 percent a year. The recession of late 1973 to 1975, when real median income of families actually fell, pulled down the average for the 1970s.

The new income figures show that, although the economy is still expanding, income improvements aren't moving nearly as fast as they were in the 1960s. At that time, there was a wide-spread expectation that living standards would simply keep rising at a substantial rate each year.

Since then, however, the vastly higher cost of imported oil, plus a slowdown in the growth of productivity for reasons which aren't yet fully understood, has placed a brake on the growth of income. Some economists believe it is no longer possible to think of taking care of all the unmet needs of the poor and sick while still improving living standards for the rest of society.

Poverty figures reflect this: the decline in the proportion of poor has virtually stopped.

In its annual survey of income and poverty, the bureau also reported that the 1978 total of persons below the poverty line ($6,662 for a non-farm family of four) declined only slightly -- by 200,000 -- from 1977. It was 24.5 million in 1978 -- 11.4 percent of the population.

The overall figures mask major differences among social groups and among regions.

For example, while overall median family income was $17,640, it was higher than that for whites -- $18,370 -- and substantially lower for Spanish families ($12,570) and black families ($10,880).

Similarly, while the national poverty rate was 11.4 percent of all persons, for whites it was only 8.7 percent, for blacks 30.6 percent and for Hispanics 21.6 percent.

The South had the most poor -- about 42 percent of the national total, although it has only one-third of the national population.

Although real median family income has risen over the past few years after falling off sharply during the 1973-75 recession, the 1978 figure didn't reach the high achieved in 1973. In that year, just before the recession, the median was $17,683 (expressed in 1978 dollars).

The Census Bureau figures show that if you made $25,000 or more family income in 1978, you made more than about three-quarters of all U.S. families, Although 16 million families made $25,000 or more, over 41 million made less -- and a quarter of all families were below $10,000.

The family figures also show that the more education you have, the less likely you are to be poor. Among families headed by a person with at least one year of college, only 3.4 percent were below the poverty line. The figure was over 20 percent for persons headed by a person who hadn't completed elementary school.

Among certain groups, poverty levels were extraordinarily high. Black families headed by women without a husband present had poverty rates ranging from 42 percent where there was one child to 83 percent where there were five or more children. The rates were lower, though still rather high, for similar white-female-headed families.