CalcWhenUUID Generator
Every press draws 122 bits of randomness into the standard 8-4-4-4-12 shape — you would need to generate about 2.7 billion billion UUIDs before a coin-flip chance of a single repeat.
How it works
Need an ID that nothing else on Earth is using, without asking a central authority? Press the button. Developers use these for database keys, file names, idempotency tokens and test data; everyone else meets them as the long codes software prints when it wants to name something exactly once.
Batches of up to fifty come out one per line, ready to paste. Each is drawn independently from your device's cryptographic random source.
The method
Sixteen random bytes from crypto.getRandomValues, with the version nibble set to 4 and the variant bits set to 10 per RFC 4122, formatted as five hyphenated hexadecimal groups (8-4-4-4-12). The remaining 122 bits are pure randomness.
Frequently asked questions
Can two generated UUIDs ever collide?
In principle yes, in practice no: with 122 random bits, collisions need on the order of 2.7 quintillion UUIDs for a 50% chance — the birthday bound. Systems generating a billion a second would wait years for even odds of one clash.
What is the difference between a UUID and a GUID?
None in substance — GUID is Microsoft's name for the same 128-bit identifier. The v4 random format generated here is accepted everywhere either name is used.
Why does the middle always start with 4?
That nibble is the version marker: 4 means “randomly generated”. The digit after it (8, 9, a or b) marks the standard variant — both are set by the format, and validators look for them.