Many software developers have misconceptions about postal codes (usually
referred to as zipcodes in the United States) and how they relate to
We hope that this guide will help cast light on some of the common
misconceptions, and thus help prevent developers from building applications
based on faulty logic.
the most common problems we see
is developers struggling when the
assumptions they made about postcodes based on their home country turn out
not to be valid in other countries.
The key point to understand is that postal codes are a tool used by postal
services to speed up the delivery of physical mail. In some parts of the
world postal codes have become a shorthand way to refer to areas, and this
has lead to a mistaken, albeit understandable, perception that postcodes
always correspond to locations.
Common points of confusion and misconceptions
Postcodes always represent locations
No, for most countries postcodes represent mail delivery points. Those
points may or may not be in fixed locations. For example, many countries
assign postcodes to military units. The postcodes stays the same regardless
of where the unit is physically based. In many countries large companies or
government departments have their own postal code with no correlation to
Postcodes never move or change
No, in most countries there is on-going fluctuations in postcodes. New
postcodes are created, old codes are retired, large organization move and
take their code with them.
Occasionally countries will change their entire postcode system (fun!).
Within a country the area covered by a postcode is always roughly the same size
No, absolutely not. Generally postcodes cover roughly the same number of people. In a
dense city that might by a very small area. In a rural area it may be massive.
Postcodes work roughly the same way in all countries
No, not at all. Almost every aspect of what a postcode is and what it means
can differ across countries. The format of the code, the number of people it
represents, etc, etc.
All countries have postcodes
No, many countries do not have any form of postal code at all.
Also, some countries technically have a postcode system, but it is not
Every geographic location belongs to a certain, unique postcode region.
While some countries have assigned postcode regions to all of their
territory, many others have not. Remote locations, for example in a large
forest, or lakes or other bodies of water, may not fall into any postal
Locations have a single, unique postcode
No, often large buildings will have multiple postcodes, for example if there
are multiple, large organizations based there.
Usually it will depend on how many people are at the location (and thus how
much mail typically needs to be sent there). An example: skyscrapers in
New York can have multiple "zipcodes" (as postcodes are known in the US).
Also when postcodes change people will often keep using the old postcode,
even after a new postcode
has been assigned to the location.
The area covered by a postcode is always within one state / province
In many countries the area covered by a postcode has no correspondance with
administrative hierarchies like county, state, province, etc. In Australia the postcode
famously spans not just multiple states but also timezones.
Postal systems are unique to a specific country
No, many countries and territories of the world use the postal service of
other countries. For example Liechtenstein uses the Swiss postal system.
For logistical convenience, exclaves of one country often use the postal
system of another country. For example the German exclave
Büsingen uses both the German and Swiss postal services
Postcodes are unique to a specific country
No, many countries use strings of numbers as their postal code.
So if you just send us a query like
we have no idea which country you mean.
Please always also add a country name.
Postcodes are always numbers
No, many countries like the UK, Canada, and the Netherlands have
But even in countries that use only digits in their postal codes it is best
to always treat postcodes as strings not numbers, otherwise leading zeros
can get removed by formatting software
(for example Excel). This is one of the most
common problems we see in geocoding query formatting
Postcodes are always alphanumeric
No, some countries require dashes or spaces in their codes. For example in
Poland postal codes are five digits but with a dash between the second
and third digit, for example:
So what should software developers do?
The first thing is you should understand and internalize that postcodes
not a great tool for communicating location. They should be avoided if
possible. Nevertheless, we understand that in some countries the habit of
searching by postcode or referring to locations by postcode has become
Even in that case it's important to understand the inherent limitations
and challenges of using postcodes to reference locations. Know the
potential problems you are building into your system.
If your application will be geocoding multiple countries do NOT fall into
the trap of assuming postal codes have the same characteristics in all
countries (or even that all countries have postcodes)
Final point: treat postcodes as string not numbers. You will thank us.