Learn why 40 is spelled as forty and not fourty , and how to use it in sentences. Find out the history, the hyphen rule, and the common mistakes of this number. Forty or Fourty ? The Common Misconception One of the most common mistakes that people make is confusing the spelling of the number 40 . Is it spelled forty or fourty ? Let’s take a closer look. The correct spelling of the number 40 is “ forty ”. This is the only accepted form and should always be used when referring to the number 40 , whether it’s used to describe age, quantity, or numbers in general. In addition, forty is also the only spelling used in all its derivatives, such as forty ... Is it ' fourty ' acres and a mule or 'forty ' acres and a mule? We'll help clear that up, plus teach you how to write out numbers up to 100.