What is the difference between alpha-2 and alpha-3 country codes?

Alpha-2 codes are two letters (GB, US) and are the most common, used in domains and locales. Alpha-3 codes are three letters (GBR, USA) and are more human-readable and collision-resistant, used in passports and some standards.

When should I use the numeric ISO 3166-1 code?

The three-digit numeric code (826 for the UK) is language-independent and script-independent, which makes it ideal for systems that must avoid Latin letters, such as some financial messaging and statistical datasets.

Is the alpha-2 code the same as a country's top-level domain?

Usually but not always. Most ccTLDs match the ISO 3166-1 alpha-2 code (.de for Germany), but there are exceptions like .uk (ISO code GB) and .eu, which is not a country code at all.

Do ISO 3166-1 codes ever change?

Yes. Codes are reassigned when countries form, dissolve or rename. Retired codes (like CS for Serbia and Montenegro) are reserved and not immediately reused, so historical data should be interpreted with the date in mind.

How are region groupings determined?

The region shown here follows the UN M.49 standard continental grouping (Africa, Americas, Asia, Europe, Oceania). M.49 also defines finer sub-regions, but the continental level is what most applications use for high-level segmentation.

ISO 3166-1 Country Codes

ISO 3166-1 country code reference

ISO 3166-1 assigns each country and dependent territory three standardized codes: a two-letter alpha-2, a three-letter alpha-3, and a three-digit numeric code. These codes power locale identifiers, ccTLDs, currency tables, shipping systems and statistical datasets. This reference lets you search by country name or any code form and shows all three side by side.

How it works

Each country has one of each code type, kept in sync by the ISO 3166 Maintenance Agency:

Alpha-2 — two uppercase letters (GB, US, AM). Most widely used; the basis for ccTLDs and BCP 47 region subtags.
Alpha-3 — three uppercase letters (GBR, USA, ARM). More mnemonic and less prone to collision; used in passports and ISO 4217 currency codes (the first two letters of a currency code are the country’s alpha-2).
Numeric — three digits (826, 840, 051). Identical to the UN M.49 country code; script-neutral, which is useful where letters cannot be used.

To convert between forms you simply look the country up in the registry — there is no algorithm, the mapping is a fixed table. The region column groups countries by UN M.49 continental area for coarse segmentation.

Tips and notes

The alpha-2 code is your safest default for web and locale work because it matches BCP 47 region subtags and most ccTLDs.
Watch the famous mismatches: the United Kingdom is GB in ISO but .uk as a domain; Switzerland is CH (from the Latin Confoederatio Helvetica).
For language tags, combine a language subtag with the alpha-2 region: en-GB, hy-AM, pt-BR.
Reserved or transitional codes exist (EU, UK, XK for Kosovo) — treat anything outside the official list as user-defined or exceptional.

ISO 3166-1 Country Codes

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ISO 3166-1 country code reference

How it works

Tips and notes