How to Do Factorial on a TI-84

Factorial doesn’t have its own key on a TI-84’s keypad, so the first time you need 5! it’s not obvious where to look — it’s tucked inside a probability menu, not the general math menu you’d expect.

Where it lives

On a physical TI-84, factorial is at MATH → PRB → 4:! — PRB standing for Probability, alongside nPr and nCr. It’s a postfix operator, meaning it goes after the number: select it, and the calculator inserts the ! symbol right after your cursor, so 5! reads left to right exactly like the math notation.

The syntax

Typed directly as an expression, it’s just 5!, no parentheses needed — the same way you’d type 5^2. You can also stack it after a full expression in parentheses, like (3+2)!, which evaluates the inside first, then applies the factorial to the result.

What it actually accepts

Factorial is only defined for non-negative integers — 0! is 1 by convention, but anything negative or non-whole (like (-3)! or 2.5!) throws a domain error rather than returning a value. It also grows fast enough to overflow well before you’d expect: past 170!, the result exceeds what the calculator can represent and errors out instead of returning a number.

Worked example

6! = 720 (6 × 5 × 4 × 3 × 2 × 1). It shows up constantly in combinatorics — nCr and nPr, the combination and permutation functions on the same PRB menu, are both built from factorial under the hood (nCr(n,r) is literally n! / (r! × (n−r)!)), which is why all three live next to each other.

Try it yourself

On the TI-84 calculator, factorial has its own dedicated x! key on the keypad — no menu digging required — or you can type ! directly after any expression, the same as 5!. The full syntax reference, including the exact domain and overflow limits, is in the docs.