How large of a bag can I take on an airplane?

According to FAA regulations, all carry-on bags must be safely stowed in an overhead bin or underneath a seat. The overhead space and "beneath the seat" space varies from aircraft to aircraft. The best idea is to contact the airline (or look on their web site) for the carry-on limitations. In most cases, the dimensions listed are in linear inches. To determine if your bag meets the dimension listed add the width + depth + height.