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What is a UUID?
A UUID (Universally Unique Identifier) is a 128-bit value used to identify information without relying on a central authority to avoid duplicates. A version 4 UUID — the type this tool generates — is built almost entirely from random bits, which makes the chance of two ever colliding astronomically small (you'd need to generate billions of UUIDs per second for centuries before a collision became likely).
Every UUID follows the same 8-4-4-4-12 hexadecimal pattern, e.g. 3fa85f64-5717-4562-b3fc-2c963f66afa6. The 4 in the third group marks it as version 4, and the leading digit of the fourth group (8, 9, a, or b) marks the RFC 4122 variant.
Common uses for UUIDs
- Primary keys in databases, so records can merge from multiple sources without ID collisions
- Unique identifiers for API requests, sessions, or tracing IDs in distributed systems
- File names or object keys in storage systems (S3 buckets, CDNs) that need to be unguessable
- Test fixtures and mock data where you need unique but disposable IDs
Which version should you use?
| Version | Built from | Use it when |
|---|---|---|
| v1 | Timestamp + random node ID | You need the creation time recoverable from the ID itself, and don't mind an older RFC 4122 format |
| v3 | MD5 hash of a namespace + name | You need the exact same input to always produce the exact same UUID, and only need MD5-level compatibility (e.g. an older system already keyed on v3) |
| v4 | Purely random | The default choice for most new IDs — reveals nothing about when or how it was created |
| v5 | SHA-1 hash of a namespace + name | Same deterministic use case as v3, preferred over v3 today since SHA-1 is a stronger hash than MD5 |
| v7 | Millisecond timestamp + random | Increasingly the recommended choice for database primary keys — sorts chronologically like an auto-increment ID, unlike v4, which improves index locality |
v3 and v5 are deterministic: the same namespace and name always produce the exact same UUID — useful for turning something like a URL or a filename into a stable, repeatable ID without a database lookup. v1, v4, and v7 are different every time.
Frequently asked questions
How random are these UUIDs?
This tool uses crypto.randomUUID(), a browser API backed by a cryptographically secure random number generator — the same class of randomness used for encryption keys, not a simple Math.random() fallback.
Can two UUIDs ever collide?
In theory yes, in practice no. With 122 random bits, you'd need to generate around 1 billion UUIDs per second for about 85 years before the probability of a single collision reached 50%.
What's the difference between UUID v1 and v4?
UUID v1 is derived from the current timestamp and a node identifier, which normally means a real MAC address that can leak information about the generating device. This tool generates the node ID as random bytes instead (with the "multicast" bit set, the standard way to mark it as not a real MAC, per RFC 4122 §4.5) — so you get a recoverable timestamp without leaking real hardware identity. UUID v4 is purely random and reveals nothing about its origin at all.
What's the difference between v3 and v5?
Both are deterministic: hash a namespace UUID plus a name, and you always get the same output UUID back. The only difference is the hash — v3 uses MD5, v5 uses SHA-1. Since MD5 is considered weak for security purposes, v5 is the generally recommended choice today; v3 mainly exists for compatibility with older systems built before v5 was common.
What is a UUID v7, and why decode a timestamp out of it?
UUID v7 (standardized in RFC 9562) starts with a 48-bit Unix millisecond timestamp, so IDs generated later always sort after ones generated earlier — good for database index locality, unlike v4's pure randomness. Because that timestamp is embedded directly, it's also fully recoverable: paste any v1 or v7 UUID into the decoder above and this tool extracts its creation time.
Is my data sent to a server?
No. UUIDs are generated entirely in your browser using the Web Crypto API — nothing is transmitted anywhere.