REALLY stupid question re: measuring suitcase

[NCDisneyMom]

Hi all! I will be flying to WDW solo in a few weeks. I have not flown in many years, so I'm kind of new to the whole flying thing. As it is just me, and I'm only staying 3 nights, I want to limit my luggage to one carry-on suitcase(along with my purse). I will be flying airtran, and they have on their website that carry-on suitcases cannot exceed 55". Does this mean that I just add up the length, width and height? Sorry if this is so stupid...I feel really stupid even asking it. I just don't want to be cramming everything into my smallest suitcase, if I can legally bring the next size up, instead. TIA!!

 

[Obi-Wan Pinobi]

Yes, the 55" will be the length + width + height.