We tend to think of postcodes as boring things that don’t change much. It turns out that there has been a large growth in the number of active postcodes over the last 40 years, as this chart shows:

Postcodes

(The interactive version allows you to hover to get information about the postcode area.)

The area with the largest number of postcodes is BT (Belfast), which covers all of Northern Ireland, and which overtook the next largest, B (Birmingham), in the 1990s. The one with the smallest number is GIR, a non-geographic code for Girobank. ZE (Lerwick) in Shetland is the geographic area with the smallest number of postcodes.

There are some interesting things buried in the data that this visualization brings out (click on the interactive version to see for yourself).

  • In 1999, postcodes for the Wirral Peninsula that previously were in the L (Liverpool) postcode area were transferred to the CH (Cheshire) area. The jumps for both these areas are very visible on the chart.
  • There are other jumps in the chart that are less easy to explain. In BS (Bristol) in 1997 the number drops dramatically for a few months, before coming back to about its previous level. According to the data, over 8000 postcodes were changed in August 1997 and in December 1997, but I can’t find any external reference to this change. It could be that it’s an administrative artifact, a blip that doesn’t mean this number of postcodes were actually removed, just that the change was spread over two separate entries. (This is just speculation though, and I could be wrong about this.)
  • NPT was a non-standard area code for Newport (NP) (since it had no district number) that was phased out in 1984. (Oddly, there doesn’t seem to be a corresponding jump in NP.)

It’s to be expected that when new houses are built more postcodes will be allocated, but there’s actually a lot more change happening than that. The chart doesn’t show it (except for a few examples like Bristol above) - but there is also a significant amount of recoding within an area - when a large number of postcodes are changed even though the number stays about the same.

For example, in Aberdeen (AB) in August 1990 the area was completely recoded, by changing the district number from 1 digit to 2 digit codes (so, for example, AB1 changed to AB1x). This change is not visible on the chart.

I spent longer than usual trying to come up with a postcode visualization. I wanted to do something that wasn’t just showing postcodes on a map - something more about the coding system itself, ideally. One early attempt was a sunburst diagram, but the sheer number of postcode areas and districts meant that it was a blur and conveyed very little information.

I think it would be interesting to visualize postcode density - HS (Hebrides) has very few postcodes over a large area, while cities like Birmingham (B) have a large number of postcodes concentrated in a relatively small area. Or perhaps area names that are most out of proportion to the size of the area they cover - some cases include SY (Shrewsbury) that extends all the way to the coast of Wales, or the small town of Llandudno that gives its name to the LL postcode area that covers much of north Wales.

Visualization type: multi-line chart

Data source: National Statistics Postcode Lookup (August 2020), Office for National Statistics (CSV, 191 MB, compressed)

Technical notes: generated using d3 and its multi-line chart; code

See also: