What is a version 4 UUID?

One where almost every bit is random, apart from a few that record the version and variant. Other versions derive from a timestamp and MAC address, or from hashing a name. Version 4 is the one to use when you simply need a unique value.

Can two UUIDs be the same?

In principle yes, in practice no, provided the randomness is real. The probability is small enough that hardware failure is a far more likely cause of a duplicate than the mathematics.

Should I store a UUID as text or as binary?

Binary, if the database supports it. The text form is 36 characters; the value is 16 bytes. On a large table that difference shows up in index size and in every join.

Are these generated on a server?

No. They are generated in your browser, so nobody else ever sees them — which is the only sensible arrangement for a value you may be about to use as an identifier.

UUID Generator

Generates version 4 UUIDs using your browser's cryptographic random source, in bulk, with the formatting options most databases and languages expect.

Runs in your browser — nothing is uploaded

Options

How many

Up to 1,000 at a time.

Version 4 UUIDs from your browser's cryptographic random number generator — not Math.random, which is not suitable for identifiers.

5 UUIDs

f0bb2e70-cda7-4c69-b457-846f8f88d1f1
2800abec-cb8e-45ca-8401-7601322e40c5
7c0f7efc-8271-4c7b-bbe0-1aa0a4c48255
2090f87c-5a0d-4b95-b8c0-0802447e118d
7436296e-74e3-41e1-b6da-80a1070873c6

Need this as an API?

Why use it

Built to be genuinely useful

Real randomness

Values come from crypto.getRandomValues, not Math.random. That distinction matters the moment an identifier is used for anything a person should not be able to guess.

Bulk generation

Generate one or a few hundred at a time, for seed data or a migration, without running the page again for each.

Nothing is uploaded

It runs in your browser. Tokens, keys and customer data routinely end up in these tools, and none of it should have to cross a network to be decoded.

Free, no sign-up

No account, no daily cap, and no feature held back behind a paywall.

How it works

Three steps

1
Choose how many you need and how they should be formatted.
2
Generate.
3
Copy the list, or download it.

Why collisions are not something to plan around

A version 4 UUID has 122 random bits. The number of values that gives you is large enough that the usual comparisons stop being helpful, but the practical consequence is simple: generating a billion a second for a century leaves the chance of a single duplicate at roughly one in a billion.

This is why UUIDs can be generated independently on different machines, offline, with no coordination, and still be safe to use as primary keys. That property — not the length, and not the format — is the entire reason they exist.

The exception is a generator that is not actually random. A UUID built from Math.random or from a poorly seeded source can and does repeat, and the failure is invisible until two records collide in production. That is the reason this page uses the browser's cryptographic source.

When a UUID is the wrong primary key

Random UUIDs make poor clustered index keys. Each insert lands at a random point in the index rather than at the end, which fragments pages and slows writes on a large table. On MySQL with InnoDB this is pronounced enough to be worth designing around.

The usual answers are to keep an auto-incrementing internal key and expose a UUID externally, or to use a time-ordered identifier such as UUIDv7 or ULID, which keeps the independence while sorting roughly by creation time. Version 4 remains the right choice when unpredictability is the point.

FAQ

Questions people ask

API access

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