UUID v4 Generator

The Anatomy of UUID Version 4

A UUID (Universally Unique Identifier) is a 128-bit label designed so that any device, anywhere, can generate one independently with near-zero probability of duplication. Version 4, defined in RFC 4122 by Paul Leach, Michael Mealling, and Rich Salz, fills 122 of those 128 bits with pure randomness. The remaining 6 bits encode the version number (4) and the variant identifier. The result is a 36-character string in the format xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx, where every x is a random hexadecimal digit and y is constrained to 8, 9, a, or b.

Structure and Color Coding

The display above color-codes the three types of characters in every UUID v4. White characters are random hex digits drawn from crypto.getRandomValues(). The gold character is always "4", marking this as version 4. The teal character is the variant indicator: its two most significant bits are fixed at "10" (binary), constraining it to hexadecimal 8 through b. Every other bit in the UUID is independent, uniformly distributed randomness.

The Collision Mathematics

With 122 random bits, UUID v4 has 5.3 \u00D7 10³⁶ possible values. The birthday paradox tells us that collisions become probable when the number of generated UUIDs approaches the square root of the total space. That threshold is approximately 2.3 \u00D7 10¹⁸ (2.3 quintillion). To reach a 50% collision probability, every person on Earth would need to generate 300 million UUIDs per second for an entire year. At any realistic generation rate, UUID v4 collisions are a mathematical impossibility in practice.

Practical Applications

UUID v4 is the standard identifier format for distributed systems. Database primary keys, API request identifiers, session tokens, message queue deduplication keys, file naming, and microservice correlation IDs all benefit from identifiers that can be generated independently on any device without coordination. The 8-4-4-4-12 hyphenated format is recognized by every major programming language, database system, and cloud platform. PostgreSQL, MySQL, MongoDB, Redis, and every major ORM support UUID columns natively.

Format Variants

The standard lowercase hyphenated format is the most common. The format selector above offers five representations of the same 128 bits:

• Standard: lowercase with hyphens, the RFC 4122 canonical form
• UPPER: uppercase with hyphens, preferred by some Microsoft APIs
• No-hyphens: compact 32-character hex string for storage efficiency
• {Braces}: curly-brace wrapped, used in Windows registry and COM
• URN: the formal urn:uuid: namespace identifier per RFC 4122

Private by Architecture

Every UUID on this page is generated inside your browser using crypto.getRandomValues(), the Web Cryptography API specified by the W3C. The 16 random bytes that form each UUID never leave your device. The server delivers this page and is finished. Your browser handles every generation independently. The recent history panel stores generated UUIDs in your browser's session memory for convenience and clears automatically when you close the tab.