Straw Hat Luffy’s intelligence can be difficult to gauge. At times, he’s impulsive, bullheaded, absentminded, ignorant and unknowledgeable. However, he also shows clear signs of initiative, determination, maturity, thoughtfulness and focus. He can be viewed as either foolish or brilliant.
Many people, both within the One Piece fandom and the One Piece world, prefer to attribute Luffy’s long-term success to dumb luck and flukes. However, it's virtually impossible to get as far as Luffy has without a great deal of conscious effort. In order to properly assess how smart Luffy is, it’s important to look over his decisions, actions and accomplishments throughout the series.
The biggest indicators of Luffy’s cleverness come from his fight history. He's able to consistently go up against people stronger than himself and pull out a win. Besting superior opponents takes much more than luck or brute force; it requires a great deal of determination, creativity, adaptability and concentration on the underdog’s part.
Luffy has also been shown to be good at figuring out an opponent’s weakness and using it against them. For example, when confronting Crocodile, he realizes he has a weakness to water based on context clues; when he runs out of water, he learns to use his own blood as a viable substitute. This proves that Luffy can't just discern weaknesses, but also find creative ways to exploit them.
There are also times where he learns an entirely new skill to defeat his enemies as he fights. Against Katakuri, he practices using his Observation Haki to see into the future and anticipate attacks. Against Kaido, he weaponizes his Conqueror’s Haki to injure the supposedly-indestructible Emperor of the sea. The ability to come up with such solutions and apply them mid-battle requires good intuition and reflexes.
Besides fighting, Luffy is also good at decision-making and taking the initiative. Many people have trouble making concrete choices or declarations for fear of having them blow up in their face. Luffy overcomes his fear of failure with the determination to see his goals achieved. This drive is why he’s able to challenge big-name pirates and the World Government, and will more than likely become King of the Pirates like he always says.
It could be argued that Luffy doesn’t have to fear failure because of everything he has going for him. He has abnormal vitality, an indomitable will, an affinity for Haki, friends to fall back on and a Devil Fruit power that makes him bulletproof (something even Whitebeard can’t boast). As long as he doesn’t die or give up, he has infinite chances to achieve his goals, and he says as much.
However, much of what Luffy has going for him can also be interpreted as further signs of his intelligence. His indomitable will could be seen as an indicator that he is strong in mind as well as in body. On his journey, many characters try to talk down to him and make him feel bad for being a no-name upstart aiming to be King of the Pirates. Most people would allow this sort of naysaying and gatekeeping to get them down and weaken their resolve, but not Luffy. He knows better than to let such petty insults bother him, so he remains steadfast and maintains his course.
In the same vein, Luffy’s mastery of Haki, a manifestation of willpower, is another indicator of the good head on his shoulders. This power can only be used by those with a calm mind, unshakeable resolve and a strong sense of self. That means it won’t work for those with simple blind confidence or a fragile ego. The virtues necessary to wield Haki explain why it’s present in so many leaders according to One Piece lore.
The Gum-Gum Fruit seems powerful, but it doesn’t start out that way. Luffy makes the Devil Fruit seem strong with his numerous, varied and creative applications of its power. When Luffy first eats the Fruit, all he can really do with his now-rubber body is stretch his limbs to punch harder and from farther away. Now he can also inflate himself, slingshot himself, increase his speed and even fly.
The friends Luffy makes are indicative of the strength of his character. The loyalty he inspires in others, his crew especially, is the result of good leadership, decent morals, emotional intelligence and empathy. Even if he comes up short, either physically or mentally, he knows to rely on his friends to give him whatever support he needs.
Given the evidence, Luffy’s IQ could easily be anywhere between 90 and 130. This would make his intelligence at least average and maybe a little above average, although not quite genius-level. His morality, choices, actions and accomplishments should say as much about him. If what he does looks stupid but it works, maybe it’s not so stupid after all.
