What is a UUID?
A UUID (universally unique identifier), called a GUID in the Microsoft world, is a 128-bit value written as 32 hexadecimal digits in five groups: 8-4-4-4-12. Version 4 UUIDs are made from random numbers. 122 of the 128 bits are random, and the remaining six mark the version (the "4" at the start of the third group) and the variant (8, 9, a or b at the start of the fourth group).
Can two UUIDs clash?
In practice, no. There are about 5.3 × 1036 possible v4 UUIDs. You would need to generate around 2.7 quintillion of them to have a 50% chance of a single duplicate. This page uses crypto.getRandomValues, the browser's cryptographically secure generator, rather than Math.random.
Where UUIDs are used
They're used as database primary keys, idempotency keys for payment APIs, file names for uploads, correlation IDs in logs, and IDs in offline-first apps where two devices must create records without talking to a server. Nothing generated here is sent or stored anywhere.
Frequently asked questions
Is a GUID the same as a UUID?
Yes. GUID is Microsoft's name for the same 128-bit format.
Are these UUIDs safe to use as secrets?
They come from a secure random source, but a UUID isn't designed to be a password or token. Use the password generator for secrets.
Are UUIDs case sensitive?
No. RFC 9562 says to output them in lower case, but to accept either case on input.
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