From Longman Dictionary of Contemporary Englishoff the top of your headoff the top of your headinformalIMMEDIATELYTHINK ABOUT if you say something off the top of your head, you say it immediately, without thinking carefully about it or checking the facts Just off the top of my head, I’d say there were about 50. → top
Examples from the Corpus
off the top of your head• It was all off the top of my head.• ""How old is Chris?" "I don't know off the top of my head."• In my imagination, I can lift off the top of my head, just like a lid.• There are some good restaurants around here, but I can't tell you their names off the top of my head.• "Do you remember her name?" "Not off the top of my head."• I keep doing stuff off the top of my head.• "How much is the house worth?" "Off the top of my head, I'd say it's worth maybe $160,000." 