The word "quintillionths" uses 14 letters: hiiillnnoqsttu.
quintillionths is a valid word in this word list. For a definition, see the external dictionary links below.
Try a search for quintillionths in these online resources (some words may not be found):
Wiktionary - OneLook Dictionaries - Merriam-Webster - Google Search
Each search will normally open in a new window.