In French, the cardinal numeral determiners, that is to say, those which express a number, are invariable, except one, which is one in the feminine, as well as twenty-one. Twenty and a hundred take one s when they are multiplied and they are not immediately followed by another numeral determinant. For example 80 is written Quatre-Vingts. On the other hand, 82 is written Quatre-Vingt Deux. It is the same for 80000 which is written Quatre-Vingt Mille, because Mille is a numeral. On the contrary 80000000 is written Eighty Millions, the reason why Vingts takes a "s" is that "Million" is a name.
This application follows the rules and exceptions for writing numbers in letters.
In addition, the conversion is instantaneous and the numbers entered have thousands separators.