Today there are 1.8 million postcodes and each postcode covers an average of 15 properties. However, a single large building can be split into multiple post codes, and some high-volume organisations are allocated their own postcode.
In the US, zip codes (short for Zone Improvement Plan) were introduced in 1963, and zip+4 was introduced in 1983 (and still not used today). There are 41,682 active zip codes, although about a third of those are used for PO boxes, military or individual organisations. It's remarkable that the USA is 40 times bigger than the UK, but the UK has 60 times as many post codes!
Each zip code covers an average of 7800 people, but that varies dramatically with population density. (Texas and California have about the same number of zip codes, but California has 12 million more people.)
In New Zealand, postcodes were introduced in 2006. They are 4 digits, with 1,856 postcodes currently assigned. On average each postcode covers 2,500 people.
Fun fact: The New Zealand post recognises both English and Māori placenames, so the following are equivalent:
Māori Language Commission
PO Box 411
Wellington 6140
PO Box 411
Wellington 6140
Te Taura Whiri i te Reo Māori
Pouaka Poutāpeta 411
Te Whanganui a Tara 6140
Pouaka Poutāpeta 411
Te Whanganui a Tara 6140
