Factorial of 489

Counting with 489!

There are 2.7915 × 10^1104 different ways to arrange 489 different books on a shelf: 489 choices for the first place, 488 for the second, and so on down to one choice for the last. If you only fill the first three places, as with gold, silver and bronze among 489 runners, there are 489 × 488 × 487 = 116,213,784 possible podiums. If the order does not matter, divide by 3! = 6 to get 19,368,964 ways to choose a group of three. Choosing a pair from 489 people can be done in 119,316 ways.

In probability, if 489 names are drawn out of a hat one at a time, the chance of drawing them in one particular order is 1 in 2.7915 × 10^1104. That is why shuffled decks and passwords with many symbols are so hard to guess.

How big is 489!?

489! has 1105 digits, so it is larger than the estimated number of atoms in the observable universe (about 10^80). The digits add up to 4419, a multiple of 9, as they are for every factorial from 6! on. Stirling's formula, √(2πn) × (n/e)^n, gives a close estimate without multiplying everything out, and gets more accurate as n grows.

Prime factors of 489!

Every prime up to 489 divides 489!. Legendre's formula counts how often: add ⌊n/p⌋ + ⌊n/p²⌋ + ⌊n/p³⌋ + … for each prime p. For 489! that gives 2^483 × 3^243 × 5^119 × 7^79 × 11^48 × 13^39 × 17^29 × 19^26 × 23^21 × 29^16 × … (93 primes in all). The 119 trailing zeros come from pairing the 119 factors of 5 with factors of 2.

Shuffles where nothing stays put

A related count is the number of derangements of 489 items: orders in which no item ends up in its original place, as in a Secret Santa where nobody draws their own name. The count is the whole number nearest to 489! ÷ e, so about 36.8% of all 2.7915 × 10^1104 orders are derangements. From about four items on, that share stays close to 1 ÷ e, roughly 37%, however many items there are.

Computing 489! yourself

In Excel or Google Sheets, type =FACT(489). Spreadsheets keep only 15 significant digits, so from 18! on the result is rounded, and FACT stops at 170!, the largest factorial that fits in a standard floating-point number. In Python, math.factorial(489) returns the exact integer with all 1105 digits, because Python integers have no size limit. The gamma function extends factorials to fractions: Γ(n + 1) = n!, so (1/2)! = √π ÷ 2 ≈ 0.8862.

Why is 0! equal to 1?

There is exactly one way to arrange nothing, and the rule n! = n × (n − 1)! only works for n = 1 if 0! = 1. Defining it this way keeps formulas for combinations and series correct at the edges.