Factorial !

Example: 4! is shorthand for 4 x 3 x 2 x 1

Factorial Symbol

The factorial function (symbol: !) says to multiply a series
of descending natural numbers. Examples:

  • 4! = 4 × 3 × 2 × 1 = 24
  • 7! = 7 × 6 × 5 × 4 × 3 × 2 × 1 = 5040
  • 1! = 1

4! is usually pronounced "4 factorial", but some people even say "4 shriek" or "4 bang"

Calculating From the Previous Value

We can easily calculate a factorial from the previous one:

factorial multiply

As a table:

n n!    
1 1 1 1
2 2 × 1 = 2 × 1! = 2
3 3 × 2 × 1 = 3 × 2! = 6
4 4 × 3 × 2 × 1 = 4 × 3! = 24
5 5 × 4 × 3 × 2 × 1 = 5 × 4! = 120
6 etc etc  

Example: 9! equals 362,880. Try to calculate 10!

10! = 10 × 9!

10! = 10 × 362,880 = 3,628,800

So the rule is:

n! = n × (n−1)!

Which says

"the factorial of any number is that number times the factorial of (that number minus 1)"

So 10! = 10 × 9!, ... and 125! = 125 × 124!, etc.

What About "0!"

Zero Factorial is interesting ... it is generally agreed that 0! = 1.

It may seem funny that multiplying no numbers together results in 1, but let's follow the pattern backwards from, say, 4! like this:

24/4=6, 6/3=2, 2/2=1, 1/1=1

And in many equations using 0! = 1 just makes sense.

Where is Factorial Used?

Factorials are used in many areas of mathematics, but particularly in Combinations and Permutations

Example: What is 7! / 4!

Let us write them out in full:

7 × 6 × 5 × 4 × 3 × 2 × 14 × 3 × 2 × 1  =  7 × 6 × 5

That was neat. The 4 × 3 × 2 × 1 "cancelled out", leaving only 7 × 6 × 5. And:

7 × 6 × 5  =  210

Done!

Example: What is 100! / 98!

Using our knowledge from above we can jump straight to this:

100!98! = 100 × 99 = 9900

A Small List

n n!
0 1
1 1
2 2
3 6
4 24
5 120
6 720
7 5,040
8 40,320
9 362,880
10 3,628,800
11 39,916,800
12 479,001,600
13 6,227,020,800
14 87,178,291,200
15 1,307,674,368,000
16 20,922,789,888,000
17 355,687,428,096,000
18 6,402,373,705,728,000
19 121,645,100,408,832,000
20 2,432,902,008,176,640,000
21 51,090,942,171,709,440,000
22 1,124,000,727,777,607,680,000
23 25,852,016,738,884,976,640,000
24 620,448,401,733,239,439,360,000
25 15,511,210,043,330,985,984,000,000

As you can see, it gets big quickly.

If you need more, try the Full Precision Calculator.

Interesting Facts

Six weeks is exactly 10! seconds (=3,628,800)

Here is why:

Seconds in 6 weeks:   60 × 60 × 24 × 7 × 6
Factor some numbers:   (2 × 3 × 10) × (3 × 4 × 5) × (8 × 3) × 7 × 6
Rearrange:   2 × 3 × 4 × 5 × 6 × 7 × 8 × 3 × 3 × 10
Lastly 3×3=9:   2 × 3 × 4 × 5 × 6 × 7 × 8 × 9 × 10

 

deck of cards

There are 52! ways to shuffle a deck of cards.

That is 8.0658175... × 1067

Just shuffle a deck of cards and it is likely that you are the first person ever with that particular order.

 

There are about 60! atoms in the observable Universe.

60! is about 8.320987... × 1081 and the current estimates are between 1078 to 1082 atoms in the observable Universe.

 

70! is approximately 1.197857... x 10100, which is just larger than a Googol (the digit 1 followed by one hundred zeros).

100! is approximately 9.3326215443944152681699238856 x 10157

200! is approximately 7.8865786736479050355236321393 x 10374

 

Advanced Topic

What About Decimals?

Can we have factorials for numbers like 0.5 or -3.217?

Yes we can! But we need to get into a subject called the "Gamma Function", which is beyond this page.

Half Factorial

But I can tell you the factorial of half (½) is half of the square root of pi .

Here are some "half-integer" factorials:

(-½)! = √π
(½)! = (½)√π
(3/2)! = (3/4)√π
(5/2)! = (15/8)√π

It still follows the rule that "the factorial of any number is that number times the factorial of (1 smaller than that number)", because

(3/2)! = (3/2) × (1/2)!
(5/2)! = (5/2) × (3/2)!

Can you figure out what (7/2)! is?