Which UUID version should I use in 2024?

UUID v7 for new projects - it's sortable (better database performance) while maintaining uniqueness. Use v4 if you need maximum randomness or wider library support. Use v5 for deterministic IDs from names/URLs. Avoid v1 (exposes MAC address) unless required.

Can UUIDs ever collide?

Theoretically yes, but practically no. UUID v4 has 2^122 possible values. You'd need to generate 1 billion UUIDs per second for 100 years to have a 50% chance of one collision. The probability is so low it's considered impossible in practice.

Why are some UUIDs uppercase and some lowercase?

UUIDs are case-insensitive - 'A1B2...' equals 'a1b2...'. RFC 4122 recommends lowercase for generation but requires accepting either case. Most tools output lowercase. When comparing, normalize to one case. Databases may be case-sensitive, so be consistent.

UUID Versions Explained: v1, v4, v5, and v7 Compared

UUID Versions

UUID (Universally Unique Identifier) has several versions, each with different properties.

Version Overview

Version	Based On	Sortable	Use Case

v1	Timestamp + MAC	Partially	Legacy systems

v4	Random	No	General purpose

v5	Namespace + Name	No	Deterministic IDs

v7	Timestamp + Random	Yes	Modern databases

UUID v4 (Random)

The most commonly used version. 122 random bits.

xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx
          ^    ^
          |    Variant (8, 9, a, or b)
          Version (4)
Example: 550e8400-e29b-41d4-a716-446655440000

Pros:

Simple to generate
No coordination needed
Privacy (no embedded info)

Cons:

Not sortable
Poor database index performance

UUID v7 (Timestamp-based)

Newest version. Timestamp prefix + random suffix.

tttttttt-tttt-7xxx-yxxx-xxxxxxxxxxxx
^^^^^^^^ ^^^^
Unix timestamp (ms)
Example: 0188a5f5-c308-7b99-b5c0-7b5c1c1c1c1c

Pros:

Sortable by creation time
Better database indexing
Still globally unique

Cons:

Reveals creation time
Slightly newer (less library support)

When to Use What

Use v4 when:

You don't need sorting
Privacy is important
Simple random IDs are fine

Use v7 when:

Database performance matters
You need chronological ordering
Building time-series data

Collision Probability

UUID v4 has 2^122 possible values. The probability of collision:

After 1 billion UUIDs: ~0.00000000000000001%
You'd need 2.71 × 10^18 UUIDs for 50% collision chance
That's generating 1 billion UUIDs per second for 85 years