Factorial of 420

Counting with 420!

There are 1.1798 × 10^921 different ways to arrange 420 different books on a shelf: 420 choices for the first place, 419 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 420 runners, there are 420 × 419 × 418 = 73,559,640 possible podiums. If the order does not matter, divide by 3! = 6 to get 12,259,940 ways to choose a group of three. Choosing a pair from 420 people can be done in 87,990 ways.

In probability, if 420 names are drawn out of a hat one at a time, the chance of drawing them in one particular order is 1 in 1.1798 × 10^921. That is why shuffled decks and passwords with many symbols are so hard to guess.

How big is 420!?

420! has 922 digits, so it is larger than the estimated number of atoms in the observable universe (about 10^80). The digits add up to 3672, 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 420!

Every prime up to 420 divides 420!. Legendre's formula counts how often: add ⌊n/p⌋ + ⌊n/p²⌋ + ⌊n/p³⌋ + … for each prime p. For 420! that gives 2^416 × 3^207 × 5^103 × 7^69 × 11^41 × 13^34 × 17^25 × 19^23 × 23^18 × 29^14 × … (81 primes in all). The 103 trailing zeros come from pairing the 103 factors of 5 with factors of 2.

Shuffles where nothing stays put

A related count is the number of derangements of 420 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 420! ÷ e, so about 36.8% of all 1.1798 × 10^921 orders are derangements. From about four items on, that share stays close to 1 ÷ e, roughly 37%, however many items there are.

Computing 420! yourself

In Excel or Google Sheets, type =FACT(420). 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(420) returns the exact integer with all 922 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.