Are v4 UUIDs really unique?

With 122 bits of entropy from crypto.getRandomValues, you can generate 2^61 UUIDs before reaching a 50% collision probability. For practical purposes they are unique.

Should I use v4 or v7?

For new systems, v7. Its monotonic time prefix makes database B-tree indexes efficient without giving up uniqueness. Use v4 only when you explicitly do not want the creation time to be recoverable.

Is the Nil UUID useful?

It is a fixed constant (00000000-0000-0000-0000-000000000000) used as a placeholder or sentinel in systems that require a UUID-typed field.

UUID Generator

Version

Count (1–1000)

Casing

Using crypto.getRandomValues — 122 bits of entropy per v4.

How to use the UUID generator

Pick a version, how many IDs you want, and whether you prefer upper- or lowercase. The tool regenerates automatically whenever you change an option — or click Generate to get a fresh batch without changing settings.

Version 4 UUIDs (RFC 4122) are 128 random bits with 122 of them carrying entropy — collisions are astronomically unlikely even at massive scale. Pick v4 when you want a general- purpose unique identifier.

Version 7UUIDs (RFC 9562, 2024) embed the current Unix time in milliseconds in the first 48 bits, followed by random bytes. They sort in creation order, which makes them excellent primary keys in databases where UUIDs as indexes otherwise hurt performance. If you're choosing between v4 and v7 for a new system, v7 is usually the better pick.

The Nil UUID is a constant defined by the spec — useful as a placeholder or sentinel.

Bytes come from crypto.getRandomValues, which is a cryptographically secure PRNG seeded by the operating system. All work happens in this tab — nothing is sent anywhere.

Related tools

Generate cryptographically random v4 UUIDs or time-sortable v7 UUIDs, one at a time or in bulk.