What is the collision probability for UUID v4?

With 2^122 (approximately 5.3 x 10^36) possible values, the probability of a collision is astronomically low. Per the birthday paradox, you would need to generate roughly 2.7 x 10^18 UUIDs (2.7 quintillion) to reach a 50% chance of a single collision. Generating 1 billion UUIDs per second for 85 years yields about a 50% chance of one duplicate — for all practical purposes, UUID v4 collisions can be considered impossible.

Can I use UUIDs as database primary keys?

Yes, but with an important caveat. Random UUIDs (v4) cause B-tree index fragmentation in databases like MySQL InnoDB because new rows are inserted at random positions in the index rather than appended at the end. This leads to page splits, reduced cache efficiency, and slower queries as the table grows. UUID v7 solves this by placing a sortable timestamp first, so inserts are nearly sequential. Alternatively, use an auto-increment integer as the clustered key and a UUID as a secondary unique column for external reference.

Is a UUID always 36 characters long?

The string representation is always 36 characters (32 hexadecimal digits plus 4 hyphens in the pattern 8-4-4-4-12). However, the raw binary representation is only 16 bytes (128 bits). Stripping hyphens yields 32 characters. Some systems use Base64 encoding to compress UUIDs to 22 characters, trading human readability for compactness.

When should I NOT use a UUID?

Avoid UUIDs when: (1) you have a single-node database and auto-increment integers suffice — they are 4x smaller and faster for joins; (2) human-readable identifiers are needed (consider a short code or slug); (3) storage size is critical (UUIDs at 16 bytes per row add up quickly in tables with billions of rows); (4) you need guaranteed ordering and cannot use v7 — consider ULID or Snowflake IDs instead.

What is a ULID and how does it compare to UUID?

ULID stands for Universally Unique Lexicographically Sortable Identifier. At 26 characters (vs 36 for UUID), it includes a 48-bit millisecond timestamp followed by 80 bits of randomness, all encoded in Crockford's Base32. ULIDs are URL-safe, case-insensitive, and chronologically sortable — making them ideal for databases and log systems. The main trade-off is that ULIDs are less standardized and have fewer available implementation libraries compared to UUIDs.

Developer Tools2026-05-10

UUID Generator: How to Generate Universally Unique Identifiers Online

A UUID (Universally Unique Identifier) is a 128-bit number displayed as 36 hexadecimal characters in the pattern 8-4-4-4-12 — for example, 550e8400-e29b-41d4-a716-446655440000. When your application needs to generate identifiers without a central authority, UUIDs are the go-to solution. Our online generator produces UUIDs in multiple versions — v1 (time plus MAC address), v4 (random), and v7 (time-ordered) — with a single click. Unlike auto-increment integers that reveal record counts and collide across distributed nodes, UUIDs can be generated independently on any server, in any browser, or on any device with a collision probability so low that generating 1 billion UUIDs per second for 85 years would still give you only a 50% chance of a single duplicate (for v4, with 2^122 random bits). This makes them ideal for distributed systems, multi-tenant databases, and client-side ID generation. Our tool runs entirely in your browser using the Web Crypto API for cryptographically secure randomness — no data leaves your machine.

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UUID Generator

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Step-by-Step Guide

Select Your UUID Version

Choose between UUID v1 (timestamp plus MAC address, useful when you need chronological ordering), v4 (fully random, the most widely used version), and v7 (time-ordered with random suffix, RFC 9562, gaining adoption in databases for index-friendly sequential ordering). A brief description explains each version's trade-offs so you can pick the right one for your use case.

Generate One or Many UUIDs

Click "Generate" to produce a single UUID instantly. Need more? Set the batch count to generate up to 100 UUIDs at once — the output appears as a bulleted list, comma-separated values, or a JSON array. Each UUID is generated independently using cryptographically secure randomness via your browser's crypto.getRandomValues() API.

Copy, Export, or Bulk Download

Click the copy icon next to any UUID to copy it to your clipboard, or use "Copy All" for batch output. For bulk generation, download the results as a .txt file or a JSON array. The tool displays UUIDs in both standard format and as raw hex (without hyphens), which some databases prefer for storage.

Tips & Best Practices

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UUID v4 uses 122 bits of randomness, giving 5.3 x 10^36 possible values — or about 2^122. Your chance of a single collision is lower than Earth being hit by a rogue asteroid in the next five seconds.

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UUID v7 (RFC 9562) is the new recommended standard for database primary keys. It encodes a 48-bit millisecond timestamp in the first 6 bytes, making it naturally sortable — preventing the B-tree index fragmentation problem that plagued random UUID v4.

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Avoid UUID v4 as a clustered primary key in databases like MySQL/InnoDB or SQL Server: the randomness causes page splits and B-tree fragmentation. Either use v7, or pair a UUID with an auto-increment integer primary key and add the UUID as a secondary unique column.

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GUID and UUID are the same thing. Microsoft calls them GUIDs (Globally Unique Identifiers), while the IETF standard (RFC 9562) calls them UUIDs. They share the identical 128-bit structure and format.

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The 36-character UUID string is 4x larger than an 8-byte bigint and 2.25x larger than the raw 16-byte binary representation. For production databases, store UUIDs as BINARY(16) instead of CHAR(36) to save space and improve index performance.

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ULID (Universally Unique Lexicographically Sortable Identifier) is a 26-character alternative that encodes a timestamp in the first 10 characters and randomness in the remaining 16. It uses Crockford's Base32, making it URL-safe and shorter than a UUID. Prefer ULID when you need sortability and human readability. Our tool can generate ULIDs alongside UUIDs.

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Never use UUID v1 if you need to keep the generating machine's MAC address private — the last 48 bits of a v1 UUID encode the network card's MAC address. Use v4 or v7 instead for privacy-sensitive applications.

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When using UUIDs in JavaScript, crypto.randomUUID() is natively supported in all modern browsers and Node.js 19+. It always produces UUID v4. For v7, use a library like uuid.js or the crypto.randomUUID() polyfill that supports the version parameter.

Frequently Asked Questions

UUID v1 uses the generating machine's MAC address and the current timestamp — it is unique but reveals when and where it was generated. UUID v4 uses 122 bits of random data and reveals nothing about its origin, making it the most popular version. UUID v7 combines a 48-bit millisecond-precision timestamp (first 6 bytes) with 74 random bits — it is both sortable and privacy-preserving, recommended for database primary keys as of RFC 9562 (2024).

UUIDs solve the distributed ID problem elegantly — 128 bits, no central coordination, and a collision probability measured in astronomical odds. Whether you choose v4 for privacy, v7 for database index performance, or ULID for compact sortability, our free generator produces exactly what you need in whatever format and quantity your project demands. Generate your UUIDs now, no account required.

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