Airline passengers could be subject to more stringent carry-on baggage regulations. On June 9, International Air Transport Association (IATA) announced new guidelines to better accommodate carry-on bags on board aircrafts with at least 120 seats.

In an effort to free up packed overhead bins, the policy suggests optimal carry-on baggage size is 21.5 inches tall by 13.5 inches wide by 7.5 inches deep. The new size is smaller than current limits imposed by many airlines. American Airlines, Delta Airlines and United Airlines currently accept bags up to 22 inches tall by 14 inches wide by 9 inches deep, while Southwest and Virgin America permit sizes up to 24 inches tall by 16 inches wide by 10 inches deep.

“The development of an agreed optimal carry-on baggage size will bring common sense and order to the problem of differing sizes for carry-on bags. We know the current situation can be frustrating for passengers. This work will help to iron out inconsistencies and lead to an improved passenger experience,” said Tom Windmuller, IATA’s senior vice president for airport, passenger, cargo and security.

Rules may vary according to airlines, which have the choice to opt-in or out of the new initiative. Details of how the baggage guideline will be implemented are also unclear and could vary among different carriers. It also raises the likelihood of having to check in bags, which typically costs $25 each way.

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At least eight international airlines will soon introduce the carry-on baggage policy. These include Air China, Avianca, Azul, Cathay Pacific, China Southern, Emirates, Lufthansa and Qatar Airways. U.S. carriers have yet to accept the new requirements.

In a statement, IATA said it is currently developing a new model with several luggage manufacturers, including Okoban, a baggage tracking solutions provider. The new bags will carry a “Cabin OK” label with IATA and Okoban logos, with an identification code indicating that the carry-on baggage meets specifications. They are expected to be in stores by the end of the year.