One is the category, the other is a brand name
Most countries call the code at the end of an address a postal code. The United States calls its version a ZIP code because that was the program name: USPS's Postal Facts page calls it the Zoning Improvement Plan, launched in 1963, and the USPS Office of Inspector General's history credits Postmaster General J. Edward Day with backing the plan. The same OIG paper uses the alternative expansion "Zone Improvement Plan" and traces the idea to the two-digit local zone numbers the Post Office Department began using in large cities in 1943.
So the honest answer to "postal code vs ZIP code" is that there is no conflict. A US ZIP code is a postal code. The difference that matters to you is format: what the code looks like and how many characters a form will accept.
Three national formats side by side
The three systems English-speaking users run into most often look nothing alike. The table compares them on the points that decide whether a form, a shipping label or a database column will accept your input.
US, Canadian and UK postal code formats compared (sources: USPS, Canada Post, Canada and UK postcode reference articles, UK Postal Museum)
| United States (ZIP code) | Canada (postal code) | United Kingdom (postcode) | |
|---|---|---|---|
| Introduced | 1963 (ZIP+4 in 1983) | Test in Ottawa, 1 April 1971; national rollout 1972-1974 | 1959 trial in Norwich; national rollout from Croydon 1966, completed 1974 |
| Characters | 5 digits, or 9 with ZIP+4 | 6, letters and digits alternating | 6 to 8 including the space, letters and digits |
| Pattern | NNNNN or NNNNN-NNNN | A1A 1A1 (space in the middle) | Outward code + space + inward code |
| What the first part means | First digit is a broad region, 0 in the Northeast to 9 in the West | First three characters: Forward Sortation Area | Outward code: postcode area and district |
| What the second part means | Last two of five digits: delivery unit or small post office; +4 narrows to a street segment or building side | Last three characters: Local Delivery Unit | Inward code: sector and unit |
| Letters allowed? | No | Yes | Yes |
What the extra four digits actually do
USPS added four digits in 1983. Its Postal Facts page says the extra numbers let mail be sorted to a specific street, and that in 1991 two more digits were added for sorting to an individual residence or business. The USPS Office of Inspector General's history of the code describes the nine-digit form as identifying "a side of a street or an office building."
You almost never need to supply them. The same OIG paper says the average customer only needs the five-digit code for mailing; the longer forms mainly help bulk mailers who presort for postage discounts. If a form has a single box marked "ZIP code" and rejects a hyphen, five digits is what it wants.
How Canada and the UK got their codes
Both countries arrived at letters-and-numbers codes for the same reason the US adopted ZIP codes: machine sorting. The UK Postal Museum describes the 1959 Norwich experiment as a response to post-war mail growth, when the General Post Office decided a national coding scheme was needed so mail could be sorted automatically. Norwich's original format was "NOR" plus a two-digit number and a single letter. The Postal Museum notes that lessons from Norwich led to a finer division of the last three characters, and the national programme began at Croydon in 1966 and was finished in 1974.
Canada came later. Contemporary newspaper reports, as compiled in the Postal codes in Canada reference article, date the first test to Ottawa on 1 April 1971, followed by Manitoba and a gradual rollout from 1972 to 1974. Canada Post today describes the result as a six-character code in the form "ANA NAN," where A is a letter and N a digit.
ZIP boundaries are not city boundaries
In the US at least, the code describes a delivery route, not a municipality. USPS's own guidance to local governments states that ZIP Code assignments follow mail volume, delivery area size and topography, "but not necessarily" municipal or community boundaries. That is why an address can carry the name of a neighboring town, a problem covered in detail in our guide to a ZIP code that shows the wrong city, and why a single city can spread across counties, as the Atlanta ZIP code list shows.
USPS also limits how often a boundary can change. Once a municipal request to move a ZIP boundary has been granted, it will not consider another request for that same boundary more often than once every 10 years, and private individuals or companies have no standing to request one.
Which one to enter on a form
Use this as a quick decision path when a checkout or sign-up form asks for a postal code:
1. Card or address is in the US: enter the five-digit ZIP code. Add the four-digit extension only if the form has a separate box for it or explicitly accepts NNNNN-NNNN.
2. Address is in Canada: enter all six characters. If the field rejects your entry, try removing the space between the third and fourth characters, since some US-built forms do not expect one.
3. Address is in the UK: enter the full postcode with the space between the outward and inward parts.
4. The form only accepts five digits but your address is outside the US: the form was built for US addresses only. Look for a country selector before typing a placeholder, because a fake ZIP can fail card address verification.