A postcode district is meant to be a convenient slice of the country, something a delivery round can cope with. Nobody told Shetland. We measured the span of all 2,768 districts, taking the straight-line distance between each district's two furthest-apart postcodes, and the results run from the absurd to the microscopic.
The giants
| Rank | District | Post town | Span | Postcodes |
|---|---|---|---|---|
| 1 | ZE2 | Shetland | 99.1 miles | 319 |
| 2 | PH33 | Fort William | 90.6 miles | 444 |
| 3 | IV27 | Lairg | 58.8 miles | 191 |
| 4 | KW17 | Orkney | 54.7 miles | 231 |
| 5 | HS2 | Isle of Lewis | 49.2 miles | 364 |
| 6 | KW14 | Thurso | 42.2 miles | 459 |
ZE2 is the undisputed champion. It covers the northern and southern ends of the Shetland archipelago at once, and its two furthest postcodes are 99.1 miles apart, roughly the distance from London to Coventry. Post from one end of the district to the other travels by ferry. PH33, based on Fort William, runs a close second at 90.6 miles because it swallows most of Lochaber, including Ben Nevis and some of the emptiest country in Britain.
All six giants are Scottish, all six are islands or Highlands, and between them they average barely one postcode per ten square miles. Delivery drivers there do not think in streets. They think in weather.
The miniatures
At the other end of the ruler, the smallest genuinely geographic districts in Britain are all in central London. NW1W, around Euston, packs 1,040 postcodes into a span of about a quarter of a mile. EC3M, the insurance quarter around Lloyd's of London, fits 82 postcodes into 400 metres. WC1R covers little more than Gray's Inn, W1F is essentially Soho east of Regent Street, and WC2E is more or less Covent Garden and nothing else.
The contrast bears spelling out. ZE2 is roughly 400 times wider than NW1W, yet NW1W contains three times as many postcodes. One postcode in the City can serve a single floor of a single building; one postcode in Shetland can serve a scatter of crofts with a mile of moor between them.
The districts with no size at all
Six districts in our data have effectively no geographic size whatsoever, with every postcode registered at a single point. These are the system's ghosts: non-geographic districts such as N1P and EC1P that exist for PO boxes and business reply mail rather than for any place you could stand in. Several hundred postcodes, no streets, no residents, span zero. They deliver perfectly well for a district that does not exist.
How we worked it out
For each of the 2,768 districts with at least 30 geocoded postcodes we took the bounding box of its postcode coordinates and measured the diagonal, which is the straight-line span between its most distant corners. Spans below 150 metres were classed as non-geographic. Explore any district's map and postcode list from its page, starting with the champion, ZE2.