What a UUID is
A UUID — Universally Unique Identifier — is a 128-bit identifier written as 32 hexadecimal digits in five groups separated by hyphens: 8-4-4-4-12, like 550e8400-e29b-41d4-a716-446655440000. (Microsoft calls the same thing a GUID; the two are interchangeable.) The problem UUIDs solve is naming things without a central registry: two computers, on opposite sides of the world, with no connection between them, can each mint identifiers for years and never collide — not because anyone coordinated, but because the space of possible values is so large that random selection effectively never repeats. That is why UUIDs turn up as database record IDs, file names, session identifiers, transaction references and tracking numbers in systems that cannot afford to ask a central server "is this number taken?".
This generator creates version 4 UUIDs, the kind made from pure randomness: 122 of the 128 bits are random, with the remaining bits fixed to mark the version (the "4" at the start of the third group) and the standard variant (the 8, 9, a or b at the start of the fourth group). The randomness comes from your browser's cryptographically secure generator — the same source your browser uses for security tokens — rather than an ordinary random function.
How unique is 'unique'?
Version 4 offers 2122 possible values — about 5.3 × 1036. To reach even a 50% chance of a single collision you would need to generate roughly 2.7 × 1018 UUIDs, which at a billion per second would take about 85 years of continuous generating. Put in everyday terms: the odds of two UUIDs from this page ever matching are far smaller than the odds of the computers involved being struck by lightning in the same week. That is the design bargain of the UUID — uniqueness bought with space, no permission or coordination required. It is also why a UUID is not a secret or a password: anyone can generate them, and unguessability is a side effect of the volume of random bits, not a guarantee to build security on. If you need a credential, the Password Generator's output is the right tool.
Generating in bulk, and checking your work
Set the count to anything from 1 to 50 and press Generate to mint a batch — the usual need is seeding a database, preparing test fixtures, or renaming a set of records at once. Each UUID appears on its own line, ready to paste into a spreadsheet, a SQL script or a configuration file. A fresh batch is generated every time you press the button; the previous one is gone, so copy before you regenerate. And if someone hands you an identifier and you want to confirm it is well-formed before trusting it in your own system, run it through the companion UUID Validator, which checks the format and reports its version.
Frequently asked questions
What is the difference between a UUID and a GUID?
Nothing of substance. GUID (Globally Unique Identifier) is Microsoft's name for the same 128-bit format defined by the UUID standard (RFC 4122). A GUID generated on Windows validates and works anywhere a UUID does, and vice versa.
Are the UUIDs generated here stored or unique to me?
They are generated in your browser at the moment you press the button, using its secure random source. This site never sees, stores or reuses them — and because version 4 UUIDs are random, nobody else will be handed the same ones.
Can I use these UUIDs in a database?
Yes — that is their most common use. Most databases offer a native UUID type; otherwise store the 36-character text form exactly as generated, hyphens included, so other tools recognize it.
Why does the third group always start with 4?
That digit is the version field: 4 declares 'this UUID was generated from random numbers.' The first digit of the fourth group is always 8, 9, a or b for the same reason — it marks the standard RFC 4122 variant.
Private by design: this generator runs entirely in your browser. The UUIDs you generate are created on your device and are never uploaded, stored or shared.