UUID Generator: Understanding Universally Unique Identifiers
Learn what UUIDs are, how UUID v4 works, and when to use them over auto-increment IDs. Generate UUIDs instantly with our free online tool.
February 12, 2026
What Is a UUID?
A UUID (Universally Unique Identifier) is a 128-bit identifier that is guaranteed to be unique across space and time. UUIDs follow a standardized format defined by RFC 4122 and are represented as 32 hexadecimal characters separated by hyphens in the pattern 8-4-4-4-12. A typical UUID looks like this: 550e8400-e29b-41d4-a716-446655440000.
The probability of generating two identical UUIDs is so astronomically small that it is practically zero. If you generated one billion UUIDs per second, it would take approximately 85 years before you had a 50% chance of creating a single duplicate. This makes UUIDs ideal for distributed systems where coordination between components is impossible or impractical.
UUID Version 4: Random UUIDs
There are several versions of UUIDs, but UUID v4 is the most commonly used in modern applications. UUID v4 is generated using random or pseudo-random numbers. Of the 128 bits, 122 are randomly generated, while 6 bits are fixed to indicate the version (4) and variant.
The Structure of a UUID v4
A UUID v4 string like f47ac10b-58cc-4372-a567-0e02b2c3d479 has a specific structure. The third group always starts with the number 4, indicating version 4. The fourth group always starts with 8, 9, a, or b, indicating the variant. All other characters are randomly generated hexadecimal digits. This structure allows you to quickly identify a UUID v4 by looking at these specific positions.
Use Cases for UUIDs
Database Primary Keys
UUIDs are increasingly popular as primary keys in databases, especially in distributed systems. Unlike auto-increment integers, UUIDs can be generated independently by any node without coordination. This makes them perfect for microservices architectures, offline-first applications, and systems that need to merge data from multiple sources.
Distributed Systems
In distributed computing, multiple servers or services often need to create records independently. With auto-increment IDs, you need a central authority to assign IDs, which creates a bottleneck. UUIDs eliminate this bottleneck entirely since any node can generate a UUID with virtual certainty that it will be unique.
API Resources
UUIDs are commonly used as resource identifiers in REST APIs. They do not reveal information about the number of records in your system (unlike sequential IDs) and cannot be easily guessed or enumerated by an attacker.
File Names and Temporary Resources
When generating temporary files, uploads, or cache entries, UUIDs provide a simple way to create unique names without risk of collision.
Session Identifiers
While dedicated session token generators offer more security features, UUIDs can serve as session identifiers in less sensitive contexts where basic uniqueness is sufficient.
UUID vs. Auto-Increment IDs
Choosing between UUIDs and auto-increment IDs involves trade-offs that depend on your specific needs:
Advantages of UUIDs
- No coordination needed: Generate IDs anywhere without a central database.
- Merge-friendly: Combine data from multiple sources without ID conflicts.
- Security through obscurity: IDs do not reveal system information like total record count.
- Offline generation: Create valid IDs without network connectivity.
Advantages of Auto-Increment IDs
- Smaller storage: A 4-byte integer vs. a 16-byte UUID.
- Better index performance: Sequential IDs lead to more efficient B-tree indexes.
- Human-readable: Easier to communicate and debug with simple numbers.
- Natural ordering: Records are automatically ordered by creation time.
When to Choose Which
Use UUIDs when you need distributed ID generation, data merging, or enhanced security. Use auto-increment IDs when performance and storage are critical, or when you need simple, human-readable identifiers in a single-database system.
How to Generate UUIDs with Simple-Toolz
Our UUID generator tool provides instant UUID generation directly in your browser:
- Navigate to the UUID generator on Simple-Toolz.
- Click generate to create a new UUID v4.
- Copy the UUID with a single click.
- Generate multiple UUIDs at once if needed.
All UUIDs are generated using the Web Crypto API for true cryptographic randomness, ensuring each UUID meets the highest standards of uniqueness and unpredictability. No UUIDs are stored or transmitted to any server.