About 20,000 passengers a day are riding the Metrorail system's newly opened Yellow Line, the transit authority announced yesterday.

The figures--the first issued by Metro since Yellow Line service started April 30--fell short of the transit agency's earlier forecasts. Nevertheless, Metro officials cautioned that preliminary statistics are likely to be revised and they said they were encouraged by the substantial number of passengers using the Yellow Line so far.

The Yellow Line, a rail shuttle across the Potomac River, provides faster trips between some parts of Northern Virginia and the District of Columbia than are available on the Blue and Orange lines. Commuters may save up to 12 minutes by taking the Yellow Line, according to figures provided by Metro officials.

Before the new line opened, Metro officials had predicted that about 3,500 passengers would ride the Yellow Line during the busiest morning hour. They said they expected this number to climb to about 5,000 after bus service between Northern Virginia and the District is reduced in late June. The most crowded hour on weekday mornings is usually from about 7:45 a.m. to 8:45 a.m.

Data provided yesterday by Metro officials showed, however, that about 1,950 riders have been taking the Yellow Line during the heaviest morning hour. Transit specialists noted, nonetheless, that ridership is likely to increase as subway patrons become more familiar with the new line.

Richard S. Page, Metro's general manager, termed the ridership figures as "consistent with our estimates." The statistics were based on counts of passengers aboard Metro trains, taken by Metro employes on four recent weekdays.

About three-fourths of the passengers--15,000 daily--took the Yellow Line during the morning and afternoon rush-hour period--between 6:30 a.m. and 9 a.m. and between 3:30 p.m. and 6 p.m. The rest used the new line at midday or in the evening.

Page cited as noteworthy Metro's finding that 40 percent, or 2,500, of morning commuters traveled outbound on the Yellow Line from the District to Virginia. Page said the statistics indicate Metro services employment centers in the suburbs, a key issue.

According to preliminary data, virtually all morning Yellow Line passengers had previously used the Blue and Orange lines. They apparently switched to save time. Morning ridership on the Blue and Orange lines dropped by about 3,800 passengers, about the same number now riding the Yellow Line.

The chart accompanying this article, comparing inbound rush hour times between the Blue-Orange and the Yellow lines, was calculated from data provided by Metro.

The transfer time, included in the figures on the chart, was estimated for typical rush-hour trips. The actual time may be different.

The transfer time was calculated as the average interval between trains during rush-hour service. The figures include 1 1/2 minutes for transfers to the Blue-Orange lines at L'Enfant Plaza, 2 1/2 minutes for transfers to the Red Line at Gallery Place and Metro Center, and three minutes for transfers to the Yellow Line at the Pentagon.

For nonrush-hour trips, the figures may be adjusted as follows to allow for the longer transfer times: For nonrush-hour trips involving transfer to the Blue-Orange lines, 1 1/2 minutes should be added to the figures shown in the chart. For nonrush-hour trips involving transfer to the Red Line, 2 1/2 minutes should be added. For nonrush-hour trips involving transfer to the Yellow Line, three minutes should be added. During evening service after 10 p.m., transfer time may be longer.

Travel time was calculated for inbound rush-hour trips from Northern Virginia to D.C. For outbound rush-hour trips from D.C. to Northern Virginia, travel time may vary because of differences in the transfer time.

For outbound rush-hour trips, the figures may be adjusted as follows: For outbound trips involving transfer from the Blue-Orange to the Yellow Line at L'Enfant Plaza, 1 1/2 minutes should be added to figures shown in the chart. For outbound trips involving transfer from the Yellow to the Blue-Orange lines at the Pentagon, no adjustment in the figures is required. For outbound trips involving transfer from the Red to the Blue-Orange lines at Metro Center, one minute should be subtracted. For outbound trips involving transfer from the Red to the Yellow Line at Gallery Place, half a minute should be added.