The “secret formula” in SpongeBob SquarePants is intentionally kept secret as a running gag, and the show never gives an official, confirmed ingredient list. The most reliable public descriptions say it’s the recipe for the Krabby Patty and that Plankton repeatedly tries to steal it from the Krusty Krab.

What people think it is

There are lots of fan theories online, but those are not canon. Some speculate it’s something simple, while others joke that it’s weird or even “nothing at all,” but none of those have been officially confirmed by the show’s creators in the sources here.

In the show

The formula is treated like a top-secret business asset, and the joke is that Mr. Krabs guards it obsessively while Plankton keeps trying to steal it. One source says the formula is stored in a safe at the Krusty Krab, and another notes that SpongeBob once knew the combination.

If you want, I can also give you a spoiler-free fan theory roundup or a “best known clues from the show” version.