He did not know off the top of his head.

Tell me the first thing off the top of your head.

He has no hair on the top of his head.

He won't know right off the top of his head.

He could just make out the top of her head.

Then he put a hand on the top of her head.

He can't come up with anyone else off the top of his head.

He could just see the top of his mother's head, right at the front.

I really can't come up with a name off the top of my head.

He held his hand out, level with the top of her head.