Take-home pay for many federal and postal workers will dip slightly next year when the Social Security/Medicare tax rate goes up.

This year that rate was 7.05 percent of salary on amounts up to $39,600. Next year the tax goes up to 7.15 percent of salary on amounts up to $42,000.

Federal employes hired before 1984 contribute 7 percent of their salaries into the civil service retirement fund, and this year also paid 1.35 percent of salary for Medicare coverage.

Next year that contribution will rise to 1.45 percent of salary. Those workers do not pay the full Social Security tax. Next year, government workers hired since the beginning of 1984 will pay the full 7.15 percent Social Security/ Medicare tax and also contribute 1.3 percent to the civil service retirement fund.

Congress has extended the interim federal retirement program -- covering the 300,000 persons hired since January 1984 -- until April. That will give it time to work out a new pension plan for those employes, one combining Social Security, modified civil service benefits and some kind of optional tax-deferred savings plan.

Employes hired before 1984 will not be required to go into the new system. Workers hired since 1984 will have to join a new system, which Congress is still working on.

The Senate has approved two retirement plans for new civil servants, while the House Post Office-Civil Service Committee has okayed another. Senate-House conferees will meet in late January or early February to come up with a compromise.