Are these UUIDs actually unique?

Version 4 leaves 122 bits random, so you would need roughly 2.7 billion billion generated ids before a 50% chance of one repeat. In software that fits in a single database, treat the collision risk as zero and let the unique constraint catch the impossible.

Which UUID version should I use for a primary key?

v4 when ids must reveal nothing and inserts are rare; v7 or a ULID when the table is large and write-heavy, because their leading timestamp keeps index inserts sequential instead of random.

Can someone look up my data from a UUID?

Not from the id itself. A v4 UUID carries no structure a third party can read. v1, v7 and ULID do encode a creation time, so if the timestamp is sensitive, use v4.

Why would I want a seeded batch?

Deterministic ids make snapshot tests and seed data reproducible — the same seed always produces the same list, which is the one thing you normally do not want from a UUID.

Generators & Utilities Ready to use

UUID Generator

Generate UUID v4, v7, v1 or ULID identifiers in your browser. Pick a format, choose how many, then copy or download the batch.

Operation

Inspect reads the version, variant, node and (for v1/v7) the creation time baked into an id you paste.

ID format

v4 for opaque primary keys; v7 or ULID when rows should insert in creation order.

How many

Written as

Ignored for ULID, which never carries hyphens.

UPPERCASE

Seed (optional)

Set a seed to get the same batch again — useful for fixtures and tests.

ID to inspect

This tool runs entirely in your browser. Your input never leaves your device.

About UUID Generator

A UUID is a 128-bit label that lets two systems agree on the identity of a record without asking a central authority for the next number. This tool mints them in your browser — what you generate is never sent to a server, stored, or seen by us.

Version 4 is the default and the right choice most of the time: 122 of its 128 bits are random, collisions are effectively impossible, and the id leaks nothing about when or where it was created. Choose v7 or a ULID when the ids will be the primary key on a busy table. Both start with a millisecond timestamp, so new rows append to the end of the index instead of scattering writes through it — in practice that is the single biggest performance difference between the formats. v1 embeds a 100-nanosecond timestamp plus a node id and is here mainly for legacy systems.

Ids come from the browser CSPRNG, so another click really does give you another batch. Set the optional seed for the opposite behaviour: a reproducible run for fixtures, snapshots, or a test that has to fail the same way tomorrow. Output can be written hyphenated, as 32 bare hex digits, with a urn:uuid: prefix, wrapped in braces, or uppercased, and any batch downloads as a text file.