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.

What is the UUID v4 Generator?

Free UUID v4 generator — create one or thousands of random RFC 4122 version-4 UUIDs with uppercase and brace options. Runs entirely in your browser, nothing is uploaded. It runs free in your browser on Gera Tools, with nothing uploaded.

UUID v4 Generator — Gera Tools

Name: UUID v4 Generator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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 for use in .NET or C# contexts where GUIDs are typically written as {xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx}.

UUID v4 anatomy

A generated UUID v4 looks like:

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

Position	Hex digits	Meaning
Group 1 (8 hex)	`3f50b2a1`	32 random bits
Group 2 (4 hex)	`9c7d`	16 random bits
Group 3 (4 hex)	`4e2f`	`4` = version 4; remaining 12 bits random
Group 4 (4 hex)	`8b41`	`8` = RFC 4122 variant; remaining 12 bits random
Group 5 (12 hex)	`6a2c9d0f1e73`	48 random bits

The 4 at the start of group 3 and the 8, 9, a, or b at the start of group 4 are the only fixed values — everything else is random. Total random content: 122 bits.

When UUID v4 is the right choice

UUID v4 is the most widely used UUID version because it requires nothing but a good random source. Choose it when:

You need globally unique identifiers without any server coordination or central authority.
The IDs are exposed in URLs, APIs, or user-facing contexts where you do not want to reveal creation order or sequence.
You are working with systems that specifically expect RFC 4122 format (PostgreSQL’s native uuid type, most ORMs, REST frameworks).

Consider UUID v7 if you need the identifiers to sort chronologically — v7 embeds a millisecond timestamp in the leading bits so IDs created later always sort after earlier ones. This is valuable for database primary keys because time-ordered keys insert at the end of a B-tree index rather than scattering across it.

UUID v4 versus GUID — the same thing

GUID (Globally Unique Identifier) is Microsoft’s term for a UUID. The format, bit layout, and collision properties are identical. A UUID v4 generated here is a valid GUID. The only practical difference is that Windows tools and .NET often display GUIDs wrapped in braces ({...}), which is why this generator has an optional brace toggle.

Collision probability — why no registry is needed

With 122 random bits, the probability that any two generated UUIDs are the same is about 1 in 5.3 × 10^36. Even generating one billion UUIDs per second for 100 years would produce a collision probability below 1 in 10^12. In practice, UUID v4 is treated as collision-free for all real-world applications. No central registry, database check, or coordination between generators is needed.

Notes

Everything runs in your browser — nothing is uploaded. The cryptographic random source (crypto.getRandomValues) is seeded by the operating system, not by Math.random, so the randomness is suitable for security-sensitive contexts such as session tokens and idempotency keys.