A UUID (Universally Unique Identifier) is a 128-bit number used to uniquely identify a record or object in a computer system. The standard format is 32 hexadecimal digits in five groups separated by hyphens: xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx.

What is the difference between UUID v1 and v4?

UUID v1 is based on the current timestamp and the MAC address of the machine that generated it — making it sortable by time but potentially leaking hardware information. UUID v4 is randomly generated with no time or location data, making it the preferred choice for most use cases.

What is a UUID?

Q: Are UUIDs truly unique?

In practice, yes. The probability of generating two identical UUID v4 values is astronomically small (approximately 1 in 5.3 × 10^36). For any real-world application, UUID v4 values can be treated as unique.

A UUID (Universally Unique Identifier) is a 128-bit identifier used to label records in software systems without requiring a central issuing authority. Example: 550e8400-e29b-41d4-a716-446655440000

Format

A UUID is written as 32 hexadecimal characters in five groups separated by hyphens: 8-4-4-4-12. The third group's first digit indicates the version; the fourth group's first digit indicates the variant.

UUID Versions

v1 — timestamp + MAC address. Time-sortable but may leak hardware info.
v3 — MD5 hash of a namespace and name. Deterministic.
v4 — randomly generated. The most common version for most use cases.
v5 — SHA-1 hash of a namespace and name. Preferred over v3.
v7 — (newer) timestamp-based like v1 but with a random component and sortable by time.

Why Use UUIDs?

Sequential integer IDs (1, 2, 3…) require a central database to issue them. UUIDs can be generated independently by any client or service without coordination, making them ideal for distributed systems, microservices, and offline-first applications.

Generate UUID v4 values instantly in your browser: Open UUID Generator →

Frequently Asked Questions

Are UUIDs truly unique?

In practice, yes. The chance of a UUID v4 collision is approximately 1 in 5.3 × 10³⁶. For any real-world system, they can be treated as globally unique.

What is the difference between UUID and GUID?

They are the same thing. GUID (Globally Unique Identifier) is Microsoft's term for the UUID standard. The format and behaviour are identical.