Most travelers abroad can stop worrying about whether buying that additional gift will push the weight of their luggage over the amount the airlines allow you to carry free.

Beginning June 1, new free baggage allowances went into effect for most overseas flights based on the number of suitcases travelers are carrying, and their dimensions, not how much the suitcases weigh.

The problem is that not all countries and not all airlines are following the new rules, nor do they have to. The rules, approved by the Civil Aeronautics Board, apply to countries wishing to participate in the new agreement and those international carriers who file with the CAB to follow them.

"It's quite a mess," one CAB official admitted last week. He said vacationers should be very careful to find out precisely what rules apply on each airline on each leg of the planned trip, both ways.

A traveler leaving the country, whose baggage allowance is determined by the number and size of the suitcases, could find that weight counts on the return, depending on the country and the carrier.

The new international baggage system closely follows the current American domestic baggage system. Under the new rules:

First-class passengers are allowed to check free two suitcases, each limited in size to no more than 62 inches (the sum of the three dimensions of the bag), plus one carry-on bag not to exceed 45 inches.

Economy-class passengers are entitled to check free two suitcases with the total dimensions of the two not to exceed 106 inches, and neither suitcase can exceed 62 inches. In addition, a bag not exceeding 45 inches in dimensions may also be carried on board to place under the seat or in the overhead compartments.

The new rules do not apply at all to travel to Central and South America. They also do not apply in nine countries who chose not to be covered - United Kingdom, Spain, Poland, Czechoslovakia, Iran, Yugoslavia, Nigeria, the Philippines and Pakistan - unless the airlines of those countries apply to the CAB to be included. At least one carrier, British Airways, has applied and is covered.

Where the new rules don't apply, the old ones do. In those cases, coach-class passengers are entitled to carry 44 pounts free while first-class passengers can take 66 pounds; both allowances include carry-on baggage.

A new system for assessing extra baggage costs also went into effect June 1. Now, excess baggage fees are charged by the piece and not by the weight. For each extra piece or larger-than-allowed piece, a traveler will be assessed a charge roughly equivalent to one per cent of the basic economy fare, sometimes a little less.

It's going to be expensive, much more expensive than it was to carry a pound or two over. Carrying one extra piece of luggage, or a bag exceeding the allowed dimensions, to Ireland from here will cost about $30; carrying it to Western Europe can cost between $40 and $50. It will cost an extra $65 or $75 to carry an extra bag to the Middle East or Africa and $95 to take it to the Far East.

Under the old excess baggage rules, which will still apply where the new rules don't, travelers pay for excess weight at a rate of one per cent of the one-way first-class fare for each kilogram (2.2 pounds).

The excess baggage charges rules apply only for six months in order to allow the CAB to monitor the new system and evaluate consumer reaction.

The new rules are based on an agreement submitted to the CAB by carriers in the International Air Transport Association; the CBA modified the agreement to make the baggage allowance for coach passengers more liberal. The agreement was prompted by an earlier board decision, which found the present IATA resolutions establishing free baggage allowances and excess charges to be unjust and unreasonable.

The CAB expects that everything will fall into place eventually, that most other countries and carriers will abandon the long-standing practice of basing international free baggage allowances on weight and adopt the new system based on baggage pieces and size. But the interim period may be confusing, aggravating and costly for unwary travelers who don't make sure ahead of their trip what is expected of them and their baggage.