off the top of your head
[spoken] if you comment on something off the top of your head, what you say is an immediate reaction and is not a carefully considered opinion, and so it might not be correct
Last year the amount of money we put into development and support was, off the top of my head, about twenty-eight thousand pounds.
if you know something off the top of your head, you know it well and can remember it easily
OK, off the top of your head, do you know the capital of South Korea?
Easy Learning Idioms Dictionary. Copyright © HarperCollins Publishers