What is this calculator?
Generates RFC 4122 version-4 UUIDs from a seed. Unlike most generators, this one is deterministic: the same seed always produces the same identifiers, which makes it useful for reproducible fixtures and test data.
Use it when:
- You need placeholder identifiers for test fixtures or seed data.
- You want the same set of IDs to reappear on every run.
- You need a batch of well-formed UUIDs to paste into a document or database.
Intended for: Developers and testers who need reproducible identifier data.
How is this calculated?
- A seeded pseudo-random number generator is initialised from the seed you enter, which is what makes the output reproducible.
- For each identifier, 128 pseudo-random bits are drawn.
- Six of those bits are then overwritten to satisfy the standard: the version field is set to 4, and the variant field is set to the RFC 4122 layout.
- The remaining 122 bits are formatted as the familiar 8-4-4-4-12 hexadecimal grouping.
Example
Generating 2 UUIDs from seed 1.
Result: Two well-formed version-4 UUIDs, in 8-4-4-4-12 form. Re-running with seed 1 returns exactly the same pair; changing the seed returns a different pair.
Frequently asked questions
Are these safe to use in production?
No. Because they come from a seeded pseudo-random generator, they are predictable by design. Production identifiers should use a cryptographically secure source such as the platform's own UUID function.
Why make them deterministic at all?
Reproducibility. Test fixtures, documentation examples, and demo data are far easier to work with when the same seed reliably produces the same identifiers.
What makes it a "version 4" UUID?
The version nibble is fixed to 4 and the variant bits follow RFC 4122, marking it as randomly generated rather than time- or name-based. The 13th hex digit of a version-4 UUID is always `4`.
How likely is a collision?
For genuinely random version-4 UUIDs, negligible — 122 random bits. For these seeded ones, collisions are entirely determined by the seed, so treat them as reproducible test data rather than unique identifiers.
Assumptions
What this calculator takes as given:
- The transformation is performed in your browser; the text you paste is not uploaded or stored.
- Input is treated as UTF-8 text.
- The generator is seeded and deterministic — identical seed and count give identical output.
Limitations
What this calculator cannot know or does not model:
- Not cryptographically secure, and therefore unsuitable for security tokens, session IDs, or production primary keys.
- Only version 4 is produced; versions 1, 5, 6, and 7 are not supported.
References
- RFC 4122 — A Universally Unique IDentifier (UUID) URN Namespace — IETF. Defines the version and variant bit layout this generator conforms to.