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11th April 2023

How do postcodes work - postcodes explained

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Postcodes are the cornerstone of the UK postal system. Without them, delivery letters, parcels and important documents would be a long and inaccurate process. With them, the postal services can quickly identify the location of a property and make sure everything is delivered on time. 

When were postcodes introduced?

Postcodes in the UK were first introduced in London between 1857 and 1858. Postcodes were started because of the high volume of letters being sent between London’s growing population. 

They were introduced by Sir Rowland Hill, according to the Postal Museum. The issue was that, as London’s population grew, so did the number of streets. And as many streets had the same or very similar names, getting messages across the city was a bit hit and miss.

Therefore, the city was broken into 10 postal districts based on their geographical location. This is why London postcodes, even today, usually start with a compass reference – N1, SE24, E17, W14. 

After the success in London, postcodes were introduced to the rest of the UK in the 1860s. These took the either the first letters of a city – M for Manchester – or the first letter along with another letter – BS for Bristol. 

After the First World War, there was a boom in letter sending. This led to the postcode system in London becoming more focussed with the numbering added – hence N1. This was rolled out to Glasgow in 1923. 

But the UK-wide postcode system wasn’t fully introduced until 1959. 

Why do you need a postcode?

Postcodes allow your item to be processed quickly and accurately by pinpointing exactly where it needs to go. 

A single postcode is not unique to one address - usually it covers a small group of addresses. Each property within that postcode will have a number or name to help identify it. 

Using the correct postcode allows your mail to go through the Royal Mail’s automated sorting machines. Mail without a postcode will have to be sorted manually and this takes much longer.

Make sure you have the correct postcode with our Postcode Finder search.

How do postcodes work?

A postcode is split into four sections, each of which narrows down the destination. 

In a central sorting offices, automated sorting machines scan the postcodes and organise the post by area. They’re then sent to the correct local sorting offices. A machine at these offices then sorts the mail based on the postcodes by street. These are actually then sorted even more, organising them by house number. 

This is why postcodes are so important – without them, the mail has to be hand sorted from the start, which takes considerably longer than the machine. 

How are postcodes decided?

We’ll use the head office postcode - YO10 5NP - to demonstrate what each section means and how postcode areas are created and decided. 

Postcode outers

The postcode outer – or outward code – is the first half of the postcode. For example: 

YO - The first one or two letters identify the postcode area - in this case the YORK area.

10 - This part is usually either one or two numbers - but some London postcodes also contain a number and letter. This is the postcode district and tells the sorting office which delivery office the mail should go to.

Inward code

The second half is the postcode inward code. 

5 - The third part of the postcode tells the delivery office which local area the mail should go to. This section usually just contains one number.

NP - The final part always contains two letters and identifies groups of up to 80 addresses. This unit code tells the delivery office which postal route will deliver the mail.

Finding the right postcode

The best way to make sure you’ve got the right postcode for your delivery is using our Postcode Finder service. Here, you can input an address and find the correct postcode, or add a postcode and see what address it brings up. Sign up to today to use this search function. 


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