Generate random version 4 UUIDs, one or up to 500 at a time, and check any UUID to see its version, variant and timestamp.
UUIDs are made on your device with its secure random generator and are never sent to Ur Next Door.
A UUID (universally unique identifier), also called a GUID on Windows, is a 128 bit label written as 32 hexadecimal digits in five groups, 8-4-4-4-12, for example f47ac10b-58cc-4372-a567-0e02b2c3d479. Software uses UUIDs to name things without asking a central server for the next free number: database rows, uploaded files, API requests, orders, devices and log entries. Two systems that never talk to each other can each create IDs and still be practically certain they will not clash. The format is defined in RFC 9562, which in 2024 replaced the older RFC 4122.
There are several versions. Version 1 mixes a timestamp with the machine's network address, which leaks when and where an ID was made. Versions 3 and 5 are derived from a name by hashing, so the same name always gives the same UUID. Version 7 starts with a millisecond timestamp so new IDs sort in creation order, which suits database indexes. Version 4 is simply random, which makes it the safe default when you just need an ID that reveals nothing.
The generator takes 16 random bytes, 128 bits, from the browser's cryptographically secure random source. It then overwrites six of those bits: the first digit of the third group is set to 4, the version, and the first digit of the fourth group is forced into 8, 9, a or b, which marks the standard variant. That leaves 122 random bits. On modern browsers the tool calls crypto.randomUUID(), which does exactly this; on older ones it falls back to crypto.getRandomValues() and sets the bits itself. It never uses Math.random(), whose output can be predicted.
With 122 random bits there are about 5.3 x 1036 possible version 4 UUIDs. By the birthday rule, you would need to generate roughly 2.7 x 1018 of them before the chance of even one repeat reached 50 percent. Making a billion every second, that takes around 86 years. For any real application, a collision is far less likely than a hardware fault.
{...}.Paste any UUID into the checker, with or without hyphens, braces or a urn:uuid: prefix. It tells you whether the format is valid, which version it claims (1 to 8), and which variant it belongs to. The variant is read from the first digit of the fourth group: 8 to b is the standard layout, while other values mark old NCS or Microsoft IDs. For versions 1, 6 and 7 it also decodes the creation time, so 017f22e2-79b0-7cc3-98c4-dc0c0c07398f shows 22 February 2022, 19:22:22 UTC. The special nil UUID (all zeros) and max UUID (all f) are recognised too.
A UUID is an identifier, not a secret. Do not use one as a password or a password reset token on its own. Everything here runs in your browser, and the generated IDs are not stored or sent to Ur Next Door, so nobody else has a copy.
