informal vs off the top of head

informal

adj
  • Reflecting everyday, non-ceremonious usage. 

  • Suited for everyday use. 

  • Not in accord with the usual regulations. 

  • Not formal or ceremonious. 

  • Not organized; not structured or planned. 

off the top of head

How often have the words informal and off the top of head occurred in a corpus of books? (source: Google Ngram Viewer )