What Is a UUID?

A UUID (Universally Unique Identifier) is a 128-bit identifier defined by RFC 4122. It is displayed as 32 hexadecimal digits in five groups separated by hyphens: xxxxxxxx-xxxx-Mxxx-Nxxx-xxxxxxxxxxxx. The M digit indicates the version (1–5) and the N digit indicates the variant.

Example UUID v4: 550e8400-e29b-41d4-a716-446655440000

The defining property of a UUID is that it can be generated on any machine, by any process, at any time, and the resulting identifier will (almost certainly) never collide with any other UUID ever generated.

The Five UUID Versions

Version	How Generated	Use Case
v1	Timestamp + MAC address	Time-ordered IDs, log entries
v2	Timestamp + MAC + POSIX UID	Rarely used (DCE Security)
v3	MD5 hash of namespace + name	Deterministic IDs from names
v4	Random (122 bits)	General purpose — most common
v5	SHA-1 hash of namespace + name	Deterministic IDs (better than v3)

UUID v4: The Most Common Choice

UUID v4 generates 122 random bits (6 bits are reserved for version and variant). The result is a UUID that carries no information about when or where it was created — just randomness. This makes v4 the best choice for:

• Database primary keys in distributed systems
• Session tokens and authentication identifiers
• File names for uploaded content
• API resource IDs in REST and GraphQL APIs
• Any ID that should not reveal system information

How Unique Is UUID v4?

With 2¹²² possible values (≈ 5.3 × 10³⁶), the probability of a collision is vanishingly small. To have a 50% chance of a single collision, you would need to generate approximately 2.7 × 10¹⁸ UUIDs — that is 2.7 quintillion. At a rate of 1 billion UUIDs per second, it would take over 85 years to reach that volume. For any real-world application, UUID v4 collisions are not a concern.

UUID vs Auto-Increment: Which to Use?

	UUID v4	Auto-Increment
Size	128 bits (36 chars)	32 or 64 bits
Globally unique	Yes	Only within one database
Reveals record count	No	Yes (enumerable)
Client-side generation	Yes	No (needs DB round-trip)
Index performance	Lower (random ordering)	Higher (sequential)
Best for	Distributed systems, public APIs	Simple, single-server apps

How to Generate a UUID

Use the ToolsBox UUID Generator to generate cryptographically random UUID v4 strings in bulk. Generate 1 to 100 UUIDs with one click, copy individually or all at once. Runs entirely in your browser using the Web Crypto API — no server, no signup.

Frequently Asked Questions

What does UUID stand for?

UUID stands for Universally Unique Identifier — a 128-bit identifier standardised by RFC 4122, displayed as 32 hex digits in 8-4-4-4-12 format. Example: 550e8400-e29b-41d4-a716-446655440000.

How unique is a UUID v4?

UUID v4 has 2^122 ≈ 5.3×10^36 possible values. To have a 50% chance of a collision, you would need to generate 2.7 quintillion UUIDs. At 1 billion per second, that would take 85+ years. For practical purposes, collisions are impossible.

What is the difference between UUID v1 and v4?

UUID v1 uses the current timestamp and MAC address — time-ordered but reveals system information. UUID v4 uses 122 random bits — reveals nothing and is suitable for most general-purpose use cases including database keys and session tokens.

Should I use UUID or auto-increment ID?

Auto-increment IDs are smaller and have better index performance but are enumerable (reveal record count) and only unique within one database. UUID v4 is globally unique, safe to generate client-side, and reveals no system information — better for distributed systems and public-facing APIs.