It's 42 inches if it doesn't empty into a pool. If it empties into a pool then it's 48 inches. I am speaking for Carnival cruise line ships. I don't know about the other cruise lines. My son just turned 6 during the cruise and he was 47 inches, my 3 year old was only 40 inches and he wasn't tall enough and they were very strict about it, they measured him, and you aren't allowed to have the toddlers sit on your lap when you go down. My 6 year old absolutley loved it, he kept going back in line for more! Have a great cruise.