UUID v4 vs v7 vs GUID
All three are the same 128-bit identifier in the same 8-4-4-4-12 format. What differs is how the bits are filled - and that decides how well they behave as a database key.
| UUID v4 | UUID v7 | GUID | |
|---|---|---|---|
| What fills it | 122 random bits | 48-bit timestamp + 74 random bits | Microsoft’s name for the same standard |
| Sortable by time | No | Yes, to the millisecond | Depends on version |
| Database index behaviour | Random inserts fragment the index | Inserts append, like an auto-increment key | As per its version |
| Leaks creation time | No | Yes - the timestamp is readable | Depends on version |
| Typical use | Public IDs, tokens, anything where unpredictability matters | Primary keys, event logs, anything inserted in volume | .NET and Windows APIs |
The practical rule: use v7 for database primary keys, and v4 when the identifier is exposed publicly and its creation time should stay private.
How likely is a UUID collision?
A version 4 UUID has 122 random bits, giving about 5.3 × 1036 possible values. The risk grows with the square of how many you generate, so it is small but not negligible at extreme volumes: generating a billion UUIDs a second for a century (about 3.16 × 1018 UUIDs) works out to roughly a 61% chance of at least one collision somewhere in that set. For virtually any real application - even one issuing millions of IDs - the probability is astronomically small; problems in practice come from a broken random source, never from exhausting the space, which is why the generator above uses the browser’s cryptographic crypto.getRandomValues rather than Math.random.
Reading a UUID
| Position | Characters | Meaning |
|---|---|---|
| Overall | 36 | 32 hex digits plus 4 hyphens |
| Group pattern | 8-4-4-4-12 | The canonical hyphenated form |
| 15th character | 1 | The version digit - 4 for random, 7 for time-sortable |
| 20th character | 1 | The variant - 8, 9, a or b for the RFC 9562 layout |
Related developer tools
What is a UUID?
A UUID is a 128-bit identifier written as 36 characters in the pattern 8-4-4-4-12, such as 3f2b8c1e-4a5d-4e7f-9b0a-1c2d3e4f5a6b. UUIDs are generated independently with no central coordinator, and the space is large enough that random v4 collisions are not a practical concern.
What is the difference between UUID v4 and v7?
UUID v4 is fully random, which makes it unpredictable but poorly ordered - random inserts fragment database indexes. UUID v7 puts a millisecond timestamp in the leading bits, so v7 values sort chronologically and index far better, while keeping enough randomness to stay unguessable.
Is a GUID the same as a UUID?
Yes, in practice. GUID is Microsoft's name for the same 128-bit identifier standard that everyone else calls a UUID, and the two are interchangeable in format and length. GUIDs are sometimes displayed wrapped in braces, but the underlying value is identical.
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