Free online tool
UUID Generator
Generate v4 UUIDs one at a time or in bulk. Cryptographically random, fully offline.
What a UUID is
A UUID (Universally Unique Identifier) is a 128-bit value formatted as five hyphen-separated groups. Version 4 UUIDs are generated from random bits, which makes them collision-resistant in practice — the chance of generating the same one twice, even at astronomical scale, is vanishingly small. That's what makes them the default choice for primary keys, session IDs, correlation IDs, and any identifier you want to be safely unique without coordinating with a central authority.
How this generator works
The tool uses the browser's native crypto.randomUUID when available, and falls back to the same underlying cryptographic RNG on older browsers. Nothing is sent to a server — every ID is generated locally, which means you can safely use the output as production identifiers.
+Are v4 UUIDs really unique?
In the mathematical sense, no — but the odds of a collision are so low (about 1 in 2^122) that they're treated as unique in every serious system.
+What's the difference between v1, v4 and v7?
v1 is based on timestamp and MAC address, v4 is fully random, and v7 is a newer time-ordered variant. v4 is the safest default for privacy and general use.
Why a random 128-bit string is safe to treat as unique
A UUID is a 128-bit identifier formatted as 32 hex digits split by hyphens into five groups. Version 4, the kind generated here, fills almost all of those bits with random data rather than deriving them from a timestamp or a MAC address.
UUIDs are everywhere in distributed systems precisely because they let independent services mint identifiers without coordinating with a central counter, but that convenience comes with trade-offs around ordering, storage size, and index behavior that are worth understanding before sprinkling them through a schema.
Where the bits actually go
Out of 128 total bits, 6 are fixed to mark the version and variant, leaving 122 bits of true randomness. That gives 2^122 possible values — a number large enough that generating a billion UUIDs per second for the age of the universe still leaves collision probability effectively at zero, a consequence of the birthday paradox math that governs how fast collision odds actually grow with volume.
Not a substitute for ordering or secrecy
Because v4 UUIDs carry no timestamp component, they can't be sorted chronologically the way a ULID or a Snowflake ID can, and inserting them as primary keys can fragment database indexes that expect roughly sequential values. They're also not appropriate as unguessable security tokens for anything more sensitive than a database key, since the randomness guarantee is about collision avoidance, not about resisting a targeted enumeration attack on a small keyspace of issued values.
The formatting gotcha
Some systems expect uppercase hex digits, others lowercase, and some strip the hyphens entirely for storage as a raw 32-character string or a binary(16) column — if two systems disagree on casing during a string comparison, a UUID that should match will fail to, so normalize case at the boundary rather than assuming every consumer treats the string the same way.
Other UUID versions you'll run into
Version 1 encodes a timestamp and node identifier, which made it useful for rough chronological ordering but leaked information about when and where it was generated. Version 3 and version 5 are deterministic, generated by hashing a namespace and a name with MD5 or SHA-1 respectively, so the same input always produces the same UUID, useful for deduplicating records derived from an external key rather than needing new randomness each time.
Database index impact at scale
Because v4 values are uniformly random, inserting millions of rows keyed by UUID scatters writes across a B-tree index rather than appending to the end, which increases page splits and can measurably slow bulk inserts compared to an auto-incrementing integer or a time-ordered ULID. Many teams solve this by keeping UUIDs as a public-facing identifier while using a sequential internal key for the actual clustered index.
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