What is Sheldon's favourite number?

Why Sheldon Loves the Number 73

If you're a fan of The Big Bang Theory, you know Sheldon Cooper doesn’t do anything without a reason—especially when it comes to numbers. His favorite, as he proudly declares in the episode The Alien Parasite Hypothesis, is 73.

But it’s not just a random pick. For Sheldon, 73 is the “best number” for very specific, very mathematical reasons. It’s the 21st prime number, and when you multiply 7 and 3 (its digits), you get 21. Plus, its mirror, 37, is the 12th prime number—mirroring the symmetry. To a mind like Sheldon’s, that’s nothing short of perfection.

Interestingly, the choice of 73 isn’t just a character quirk—it’s a subtle nod from the show to actor Jim Parsons, who was born in 1973. This little easter egg adds a personal layer to what might otherwise seem like pure nerd trivia. Fans have noticed that Parsons, even off-screen, sometimes wears a t-shirt featuring the number, blending fiction and reality in the sweetest way.

The number has since become a symbol of the show’s blend of science, humor, and heart. It’s more than a detail—it’s a cultural reference that fans instantly recognize. Whether it’s a nod to prime numbers, a birthday coincidence, or just a quirky character trait, 73 lives on as one of the most iconic numbers in TV history.