UUID Generator

What a UUID is

A universally unique identifier is a 128-bit value written as 36 characters: eight, four, four, four, twelve, separated by hyphens. Version 4 is almost entirely random, which is what lets two systems that have never communicated generate identifiers that will not collide.

How unique is "unique"

A v4 UUID has 122 random bits — about 5.3 × 10³⁶ possibilities. You would need to generate roughly a billion a second for around 85 years before the chance of a single duplicate reached 50%. In practice, treat a collision as impossible and design for something else.

Where they are used

• Database keys — where rows are created in several places at once and an auto-increment column would clash.
• Distributed systems — identifiers that need no coordination between services.
• Idempotency keys — so a retried API call is not processed twice.
• File names — uploads that must not overwrite each other.

One database caveat

Random UUIDs make poor clustered primary keys in some databases, because inserts land in random places in the index rather than at the end, which fragments it. If that matters to you, look at UUID v7 or a sortable identifier such as ULID, which keep uniqueness while preserving insert order.

Frequently Asked Questions

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