A UUID (Universally Unique Identifier) is a 128-bit label used to uniquely identify objects in computer systems. It is represented as 32 hexadecimal digits arranged in the pattern 8-4-4-4-12, for example: 550e8400-e29b-41d4-a716-446655440000. UUIDs are designed to be unique without requiring a central registration authority — two independently generated UUIDs are astronomically unlikely to collide.

What is the difference between UUID v4 and v7?

UUID v4 is randomly generated — all 122 meaningful bits are random, making it unpredictable and suitable for any context where you don't need ordering. UUID v7 embeds a Unix millisecond timestamp in the first 48 bits, followed by random bits. This makes v7 UUIDs sortable by creation time, which significantly improves database index performance (B-tree indexes cluster related rows together instead of random scatter). Use v4 for security tokens and session IDs; use v7 for database primary keys.

How likely is a UUID collision?

UUID v4 collision probability is extraordinarily low. Generating 1 billion UUIDs per second for 100 years would produce approximately 3×10²¹ UUIDs — and you'd still have only about a 50% chance of finding one duplicate. In practice, UUID collisions are treated as impossible for all engineering purposes.

Should I use UUIDs as database primary keys?

UUID v7 (timestamp-ordered) is excellent for database primary keys because the sequential prefix keeps inserts clustered in the B-tree index, avoiding the random-write performance problem of v4. UUID v4 as a primary key can cause significant index fragmentation in large tables because random values scatter across the tree. If your database supports it natively (PostgreSQL uuid type, MySQL UUID_TO_BIN), prefer UUIDs over auto-increment integers when you need globally unique IDs that don't leak table row counts.

Are UUIDs secure enough to use as session tokens?

UUID v4 is generated using a cryptographically secure random number generator and has 122 bits of randomness, which is sufficient for most session token use cases. However, purpose-built token generators (like crypto.randomBytes in Node.js or secrets.token_hex in Python) are slightly preferable because they do not expose the UUID format. The UUID format is public knowledge, which means an attacker knows the exact structure — though 122 random bits remains practically unguessable.

Encoding & Conversion

UUID Generator

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Generated UUIDs

2639db30-b9fd-4233-9804-ac73a0f59c91

UUID — frequently asked questions

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