How does a URL shortener work?

Have you ever wondered links like,, ?

They are some of the renowned URL shortening services on the Internet.

This aim of this post is to explain how does URL shortening works, so one can make on his own.

This post deals with the concepts invovled. If you are looking for code, click the lord’s face and get it.

Talk is cheap, show me the code


  • Interest in topic.
  • Base conversion. If you know to convert deciman to hexadecimal and back, that’s more than enough.

What is a shortened URL?

Consider the following examples:

Shortened version Original URL

The shortened versions appear more precise to the original ones.

There are two major use-cases where shortening URLs is helpful.

  • Save space Targets like SMS, tweets have strict character-count limit where long URLs can take up lot of space.

  • Counting clicks This is the real and most important use of URL shortening. Consider the third link in above table. Every time one clicks on the link, Twitter gets informed and redirects you to the original link.

Finding a short representation

Well, that’s simple. Index it.

Just associate a serial number to any content, and that gets indexed. Isn’t this how we are allotted roll numbers in schools?

Here’s how it might look like:

Serial # URL

But that’d eventually lead to a problem.

There will come a time soon when the numbers used will be of more than 8 or 10 digits, which is fine but can be made even better.

A higher base would help

Decimal number system use base 10, i.e. they use 10 symbols to represent a number. Hexadecimal system uses 16 (0-9 A-F). Thus, a large number like 65432 is represented as ff98.

Take even higher base, say 26 (0-9 a-p), 36 (0-9 a-z), 62 (0-9 a-z A-Z), and apply the same concept.

Here’s the conversion that’d happen:

Base 10 Base 16 Base 26 Base 36 Base 62
27 1b 11 r r
938641 e5291 21adf k49d 3Wbn
999999999 3b9ac9ff 3647job gjdgxr 2wDOpb
987948554334 e60652985e 4j030eice cluv27a6 NOolhwe

For an exercise,

3Wbn =>  (3 * 62^3)  + (W * 62^2)  + (b * 62^1) + (n * 62^0)
      = (3 * 238328) + (58 * 3844) + (11 * 62)  +    23
      =   714984     +   222952    +   682      +    23
      = 938641

Clearly, bases 36 and 62 seem to be safest choices and require less space compared to smaller bases.

So how does this work in software?

Find code implementation here

  • Assign a serial number to a URL. This can be the primary key if it is a SQL database.
  • Convert that serial number to base 62 (you can use any base).
  • is the required URL.

  • Whenever one hits the above URL, you get the string for base_62_value.
  • Convert that base_62_value to base_10 integer as above exercise does.
  • That is the serial number you are looking for.


  • Make sure the serial number is something unique and never repeats.
  • If you are using MongoDB, use a serial number stored in some other collection.

:tada: Here you go! Make one like this yourself.

#concept #programming #urlshortener #algorithm

Written on June 18, 2018