Reading a UUID
A UUID is written as 32 hexadecimal digits in five groups: 8-4-4-4-12. The first digit of the third group is the version. In 550e8400-e29b-41d4-a716-446655440000 it is 4. The first digit of the fourth group indicates the variant (8, 9, a or b for standard UUIDs).
Version 4: random
Version 4 UUIDs are 122 random bits plus the version and variant markers. They reveal nothing about when or where they were made, which is good for public identifiers. Their randomness has a cost in databases, though: new rows are inserted at random positions in the index, which can slow down large tables.
Version 7: time-ordered
Version 7, standardised in RFC 9562 in 2024, starts with a 48-bit Unix timestamp in milliseconds followed by random bits. UUIDs created later sort after earlier ones, so database indexes stay compact and inserts are faster — similar to an auto-increment number, but still unique across servers.
The trade-off: anyone can read the creation time from a v7 UUID.
Which to choose
| Use | Recommended |
|---|---|
| Database primary keys | v7 |
| Event or log IDs that should sort by time | v7 |
| Public IDs where creation time should stay private | v4 |
| Test data, file names, general use | v4 |
Generate and inspect UUIDs
- In the UUID Generator, choose version 4 or version 7.
- Enter how many you need — up to 1,000 — and pick a format, such as upper case, without hyphens or with braces.
- Select Generate UUIDs, then copy them or download a text file with one per line.
- To check an existing ID, paste it into Inspect a UUID to see whether it is valid, its version and variant, and the timestamp encoded in versions 1, 6 and 7.
Random values come from your browser’s cryptographically secure random number generator.
Frequently asked questions
Can two UUIDs ever be the same?
In theory, yes; in practice, the probability for properly generated v4 or v7 UUIDs is so small that it can be ignored.
Is a GUID the same as a UUID?
Yes. GUID is the name Microsoft uses for the same 128-bit format.