I don't know it top off my head but I do know about it.
Any correction needed?
The standard phrase is this. I don't know it off the top of my head. Clive
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The standard phrase is this.
I don't know it off the top of my head.
Clive
"I don't know it top off my head" means that you cannot remember a specific bit of information exactly right then. "I do know about it" means that you are at least somewhat familiar with a topic in a general way. The two ideas do not go together. If you remove "about", they do: "I don't know it top off my head, but I do know it." I added the obligatory comma.