What a UUID is
A UUID (universally unique identifier) is a 128-bit value, usually written as 32 hexadecimal digits in five groups separated by hyphens, such as f47ac10b-58cc-4372-a567-0e02b2c3d479. This tool generates version 4 UUIDs, which are built from random bits rather than from a timestamp or a machine identifier, using your browser's built-in cryptographically secure random number generator.
Why they don't collide in practice
A version 4 UUID has 122 random bits (the other 6 are fixed to mark the version and variant). The number of possible values is so large that generating billions of UUIDs still makes an accidental duplicate astronomically unlikely. Two people generating UUIDs at the same moment, on different computers, with no communication between them, will still get different values.
Formats
Choose the standard format with hyphens and lowercase letters, a version with the hyphens removed, or an uppercase version. The format only changes how the same underlying random value is displayed; it does not change how the UUID was generated or how random it is.
Generating many at once
You can generate up to 1,000 UUIDs in a single batch. Each one is generated independently, with no relationship to the others beyond each being separately random, so a batch is exactly equivalent to generating that many UUIDs one at a time.
What UUIDs are not
A UUID is an identifier, not a secret. It is not designed to be unguessable the way a password or an API key is, since its purpose is uniqueness rather than secrecy; do not use a UUID as a substitute for a properly generated secret token.