How to use
- Choose v4 for fully random IDs or v7 for IDs that sort by creation time.
- Set how many you need, from 1 to 100, plus the format and letter case.
- Press Generate new, then Copy result to copy them all, one per line.
- To check an existing ID, paste it under Check a UUID to see its version, variant and embedded time.
UUIDs are made in your browser with its secure random generator, and the checker never puts what you paste into the page link.
How a UUID works
A UUID is 128 bits, written as 32 hex digits in groups of 8-4-4-4-12 (RFC 9562). Two fields are fixed:
- the version is the first digit of the third group (4 or 7 here);
- the variant is the first digit of the fourth group: 8, 9, a or b for standard UUIDs.
Version 4 fills the other 122 bits at random. Version 7 puts the Unix time in milliseconds in the first 48 bits, then a counter and random bits, so IDs made later sort later. That keeps database indexes tidy.
Example
The RFC 9562 sample v7 UUID 017f22e2-79b0-7cc3-98c4-dc0c0c07398f reads like this:
| Part | Hex | Meaning |
|---|---|---|
| Timestamp | 017f22e279b0 | 1,645,557,742,000 ms = 2022-02-22 19:22:22 UTC |
| Version | 7 | Unix time-ordered |
| rand_a | cc3 | 12 bits: random here; this tool uses a counter to keep order |
| Variant | 9 | Standard (RFC 9562) |
| rand_b | 8c4dc0c0c07398f | The rest of the 62 random bits |
Collisions, GUIDs and when not to use a UUID
- Collisions: with 122 random bits, a billion v4 UUIDs have about a 1 in 10¹⁹ chance of containing any repeat (our birthday-bound calculation).
- GUID is Microsoft’s name for the same thing. The braces style, {…}, is .NET’s “B” format.
- Not for secrets: RFC 9562 says UUIDs must not be used as security tokens. A v7 UUID also reveals when it was made.
- Nil and max: all zeros and all ones are special values with no version.
Want to read a v7 timestamp as a date in your time zone? Paste the milliseconds into the Unix timestamp converter.