A version-4 UUID is a 128-bit identifier with 122 random bits, defined by RFC 4122. The randomness makes collisions astronomically unlikely, so it is the most common UUID for generating unique keys without coordination.

How random are these UUIDs?

They use the browser's Web Crypto getRandomValues, a cryptographically strong random source, then set the version and variant bits exactly as RFC 4122 requires.

Are the UUIDs sent to a server?

No. Everything runs entirely in your browser and nothing is uploaded — close the tab and the IDs are gone.

What is the difference between UUID and GUID?

They are the same thing. GUID (Globally Unique Identifier) is Microsoft's name for a UUID. The format and rules are identical, so a UUID v4 is also a valid GUID.

Why does every UUID v4 have a 4 in the third group?

The 13th hex digit is fixed to 4 to mark the version, and the 17th digit is always 8, 9, a or b to mark the RFC 4122 variant. The other 122 bits are random. So the pattern is xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx.

Can two UUID v4 values ever collide?

In theory yes, in practice no. With 122 random bits you would need to generate billions of UUIDs per second for many years before a collision became likely, which is why no central registry is needed.

Are UUID v4 values safe to expose in URLs or APIs?

Yes. They are random, not sequential, so they do not leak counts or ordering the way an auto-increment integer ID would. That makes them a common choice for public-facing resource identifiers.

UUID v4 Generator — Gera Tools

Random UUID v4 generator

This tool mints random version-4 UUIDs — 128-bit identifiers (defined by RFC 4122) used as primary keys, request IDs, idempotency tokens, and public-facing resource IDs. Because there is no central registry, anyone can generate one offline and the odds of a collision are vanishingly small. Generate one or up to a thousand at once.

How it works

The generator fills a 16-byte array with crypto.getRandomValues — the browser’s cryptographically secure random source, not the weaker Math.random. It then sets the version nibble to 4 (byte 6) and the variant bits (byte 8) exactly as RFC 4122 requires, and renders the bytes as lowercase hex grouped 8-4-4-4-12. Optional toggles uppercase the output or wrap each value in { } braces.

Example

A generated UUID looks like:

3f50b2a1-9c7d-4e2f-8b41-6a2c9d0f1e73

Note the 4 starting the third group (the version) and the 8 starting the fourth group (the variant) — both are fixed; everything else is random.

UUID anatomy

Position	Hex digits	Meaning
Group 3, first digit	`4`	Version 4 (random)
Group 4, first digit	`8`, `9`, `a` or `b`	RFC 4122 variant
All other digits	random	122 random bits

Everything runs in your browser — nothing is uploaded.