I think that his IQ falls somewhere between 160-180.

What’s Lelouch IQ?

Lelouch Lamperouge (IQ 210)

Even without the special power of Geass, he turns out to be a tough opponent by his genius-level intellect of IQ 210.

How smart is Lelouch?

Lelouch vi Britannia is one of the most intelligent characters to be introduced in Code Geass. Lelouch showed an advanced level of comprehension, even in his youth, and has always exceeded the expectations of those around him.

1 Lelouch Lamperouge

The protagonist and driving force of the series, Lelouch Lamperouge is one of the most brilliant strategists in anime. He is a genius tactician and is able to see several moves ahead of his opponents.