UUID / GUID Generator

What is a UUID / GUID?

A UUID (Universally Unique Identifier), also called a GUID (Globally Unique Identifier), is a 128-bit label standardised by RFC 4122. It is written as 32 hexadecimal digits in five hyphen-separated groups: xxxxxxxx-xxxx-Mxxx-Nxxx-xxxxxxxxxxxx, where M is the version digit and N encodes the variant.

UUIDs are designed to be unique across space and time without requiring a central issuing authority. A database, a microservice, or a browser tab can generate a UUID locally and be confident it will never collide with one generated anywhere else; the probability of collision for v4 UUIDs is astronomically small (roughly 1 in 5.3 × 10³⁶ per pair).

UUID versions explained

Version 4: Random (recommended for most uses)

UUID v4 fills 122 bits with cryptographically random data and sets the remaining 6 bits to encode the version (4) and variant (10xx). Because it is completely random, v4 is the simplest choice: no coordination, no time dependency, no privacy concerns. It is the most widely used UUID version. Use v4 for session tokens, API keys, database row IDs, or anything that just needs a unique string.

Version 7: Time-ordered (recommended for database primary keys)

UUID v7 (IETF draft) encodes the Unix millisecond timestamp in the most significant 48 bits, followed by random data. Because newer UUIDs sort higher than older ones, v7 is friendly to B-tree database indexes: inserts always go near the end of the index, avoiding the page splits and fragmentation that random v4 UUIDs cause in high-insert workloads. Use v7 when you need both global uniqueness and natural chronological ordering, for example as primary keys in Postgres, MySQL, or SQLite.

Version 1: Time-based (legacy)

UUID v1 encodes a 60-bit Gregorian timestamp (100-nanosecond intervals since 15 October 1582) plus a clock sequence and node ID. The original spec used the machine's MAC address as the node, which exposed hardware identity. This generator uses a random node ID instead to preserve privacy. v1 is rarely chosen for new projects. v7 provides timestamp ordering with fewer historical quirks.

NIL UUID

The NIL UUID (00000000-0000-0000-0000-000000000000) has all 128 bits set to zero. It is used as a sentinel "no value" or "not initialised" placeholder, analogous to null in typed systems. Some APIs and ORMs use it to mean "auto-assign on insert."

UUID format options

• Standard: lowercase with hyphens: 550e8400-e29b-41d4-a716-446655440000. The RFC 4122 canonical form.
• No Hyphens: 32 lowercase hex characters with no separators: 550e8400e29b41d4a716446655440000. Common in compact storage, URLs, and some ORMs.
• UPPERCASE: uppercase letters with hyphens: 550E8400-E29B-41D4-A716-446655440000. Required by some older APIs and Windows registry entries.
• Braces { }: standard UUID wrapped in curly braces: {550e8400-e29b-41d4-a716-446655440000}. The Microsoft GUID format used in COM, Windows Registry, and .NET.
• URN: prefixed with urn:uuid:: urn:uuid:550e8400-e29b-41d4-a716-446655440000. Used in XML namespaces, some RDF systems, and wherever UUIDs appear as URIs.

Frequently asked questions

What is a UUID?

A UUID (Universally Unique Identifier) is a 128-bit label formatted as xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx. It is used as a unique key in databases, distributed systems, and APIs because it can be generated on any machine without central coordination.

What is the difference between UUID v4 and v7?

UUID v4 is entirely random. UUID v7 embeds a Unix millisecond timestamp in its most significant bits so that later UUIDs sort higher, which reduces B-tree index fragmentation when used as database primary keys. Use v7 when ordering matters, v4 when it does not.

What is UUID v1?

UUID v1 encodes a Gregorian timestamp plus a node ID. The original spec used the MAC address as the node, leaking machine identity. This generator uses a random node. v1 is largely superseded by v7 for new systems.

What is the NIL UUID?

The NIL UUID (00000000-0000-0000-0000-000000000000) is a special value with all bits zero. It is used as a sentinel "no value" placeholder in APIs and databases.

Is it safe to generate UUIDs in the browser?

Yes. This tool uses crypto.randomUUID() and crypto.getRandomValues() from the Web Crypto API, which provides cryptographically strong randomness. No data is sent to a server generation is local.

UUIDv7 vs. UUIDv4 migration guide

UUIDv7 is the new recommendation for database primary keys because its timestamp prefix makes inserts sequential, drastically reducing B-tree index fragmentation in high-write workloads. UUIDv4 remains the better choice for security tokens, session IDs, and API keys where unpredictability is the primary requirement \u2014 its 122 random bits provide more entropy than v7\u2019s 74 random bits. In practice: use v7 for rows in a database, use v4 for anything that must be unguessable.

Database storage size

At one million rows, CHAR(36) wastes 20 MB per UUID column compared to BINARY(16). At a billion rows the difference is 20 GB \u2014 relevant for large-scale systems.

UUID v4 collision probability

UUID v4 has 122 random bits. The probability of a collision when generating n UUIDs is approximately n\u00b2 / 2¹²³. For practical context: generating 1 billion UUIDs per second continuously, you would expect to wait approximately 85 years before seeing a 50% chance of any single collision. For all\n practical purposes, UUID v4 collisions are not a real-world concern.