UUID Generator

A version 4 UUID is 128 bits: 122 random bits plus a fixed 4-bit version (0100) and 2-bit variant (10) (RFC 9562 §5.4). In the text form xxxxxxxx-xxxx-4xxx-Nxxx-xxxxxxxxxxxx, the version is the 4 that starts the third group, and the variant makes N one of 8, 9, a, or b. The random bits come from the browser's cryptographic random source (crypto.getRandomValues), as RFC 9562 §6.9 recommends. UUIDs are generated locally and never sent anywhere.

How likely is a duplicate? With 122 random bits, the chance that any two of n UUIDs are equal is about n² / 2^123 (the birthday bound; this is QuickKit's arithmetic, not a figure from the RFC). For one billion UUIDs that is about 9.4 × 10^-20. In practice duplicates come from a faulty random source, a copied value, or a bug, not from this arithmetic, so a database that relies on uniqueness should still enforce it, for example with a unique constraint.

UUIDs are identifiers, not secrets. RFC 9562 §8 says not to assume they are hard to guess and not to use them as security capabilities, where holding the value grants access. For a password or an access token, use a dedicated generator.

Formats: RFC 9562 §4 allows uppercase, lowercase, or mixed hexadecimal digits; this tool writes lowercase unless you choose Uppercase. Without hyphens gives the 32-digit form, and urn:uuid: gives the URN form (urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6), without braces even if braces are selected. Keep Include hyphens on for a URN: with it off, the tool currently writes urn:uuid: followed by the 32 digits, which does not match the hyphenated syntax that RFC 9562 §4 defines for URNs. Braces are a convention some software uses for GUIDs and are not part of RFC 9562. The nil UUID (all 128 bits zero, §5.9) means "no value" and is not random. Up to 512 UUIDs can be generated at a time; to check the version and variant of an existing UUID, use UUID Inspector.