Factorial of 42

Counting with 42!

There are 1.4050 × 10^51 different ways to arrange 42 different books on a shelf: 42 choices for the first place, 41 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 42 runners, there are 42 × 41 × 40 = 68,880 possible podiums. If the order does not matter, divide by 3! = 6 to get 11,480 ways to choose a group of three. Choosing a pair from 42 people can be done in 861 ways.

In probability, if 42 names are drawn out of a hat one at a time, the chance of drawing them in one particular order is 1 in 1.4050 × 10^51. That is why shuffled decks and passwords with many symbols are so hard to guess.

How big is 42!?

42! has 52 digits, so it is beyond everyday counting: it dwarfs the number of grains of sand on Earth (around 10^18). The digits add up to 189, 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 42!

Every prime up to 42 divides 42!. Legendre's formula counts how often: add ⌊n/p⌋ + ⌊n/p²⌋ + ⌊n/p³⌋ + … for each prime p. For 42! that gives 2^39 × 3^19 × 5^9 × 7^6 × 11^3 × 13^3 × 17^2 × 19^2 × 23 × 29 × … (13 primes in all). The 9 trailing zeros come from pairing the 9 factors of 5 with factors of 2.

Shuffles where nothing stays put

A related count is the number of derangements of 42 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 42! ÷ e, so about 36.8% of all 1.4050 × 10^51 orders are derangements. From about four items on, that share stays close to 1 ÷ e, roughly 37%, however many items there are.

Computing 42! yourself

In Excel or Google Sheets, type =FACT(42). 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(42) returns the exact integer with all 52 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.