UUID4 Generator

Q: What is the UUID v4 collision probability?

Vanishingly small. The probability is approximately 1 in 2^122. If every person on Earth generated 600 million UUIDs, the probability of any two being the same would be about 50%.

Q: UUID vs ULID vs Nano ID?

UUID v4 is the universal standard supported everywhere. ULID adds time-ordering and is more compact. Nano ID is URL-friendly and often used in frontend applications. For most backend and API use cases, UUID v4 remains the safest choice.

UUID v4 Format

A UUID v4 follows the pattern xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx, where each x is a random hexadecimal digit and y is one of 8, 9, a, or b.

xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx

128 bits total

A UUID is a 128-bit value, displayed as 32 hexadecimal characters (0-9, a-f) separated by hyphens into five groups (8-4-4-4-12).

Version nibble: `4`

The 13th hex digit is always 4, identifying this as a version 4 (random) UUID. This occupies bits 48-51.

Variant bits: `8-b`

The 17th hex digit is constrained to 8, 9, a, or b, marking it as an RFC 4122 variant. This uses 2 of the 128 bits.

122 random bits

After accounting for the 4 version bits and 2 variant bits, 122 bits are cryptographically random - yielding 5.3 × 10³⁶ possible UUIDs.

What Are UUIDs?

A universally unique identifier (UUID) is a 128-bit number used to identify information in computer systems.
Wikipedia →

We use IDs to identify things, and usually sequential numbers are used (e.g. #1, #2, #3). But sequential IDs require knowing the previous value and a central authority to assign them - inviting race conditions and extra coordination.

UUIDs are a popular alternative. Version 4 UUIDs are generated from 122 bits of cryptographically secure randomness, making collisions so unlikely that you're more likely to be hit by a meteorite than to generate the same UUID twice. They can be created independently by any system, at any time, with no coordination required.

BTW, want to see something cool? Someone built everyuuid.com which lists every possible UUID v4. All 5.3 × 10³⁶ of them.

UUID Version Comparison

UUID v4 is the most widely used version, but other versions serve different purposes.

	UUID v1	UUID v4	UUID v7
Basis	Timestamp + MAC address	Random	Timestamp + random
Sortable	Partially	No	Yes (time-ordered)
Privacy	Leaks MAC & time	Fully opaque	Leaks creation time
Collision risk	Extremely low	Extremely low	Extremely low
Random bits	~14	122	~62
Best for	Legacy systems	General purpose IDs	Database primary keys
Spec	RFC 4122	RFC 4122	RFC 9562

Common Use Cases

Database Primary Keys

UUIDs let you generate IDs client-side before inserting into the database, avoiding round-trips and enabling offline-first architectures.

Distributed Systems

Multiple services can independently generate unique IDs without coordination, eliminating single points of failure in ID generation.

Session Tokens

The 122 random bits in UUID v4 provide sufficient entropy for session identifiers that are practically impossible to guess.

Idempotency Keys

Attach a UUID to each API request so servers can detect and safely deduplicate retries without processing the same operation twice.

File & Resource Naming

Generate collision-free filenames for uploads, temporary files, or cloud storage objects without checking for existing names.

Testing & Fixtures

Use UUIDs in test data to ensure each test run has unique identifiers, preventing flaky tests caused by ID conflicts.

Generate UUIDs in Code

Python JavaScript Go PHP Ruby curl

import uuid

id = uuid.uuid4()
print(id)  # e.g. 7c9e6679-7425-40de-944b-e07fc1f90ae7

// Node.js & modern browsers
const id = crypto.randomUUID();
console.log(id);  // e.g. 7c9e6679-7425-40de-944b-e07fc1f90ae7

package main

import (
    "fmt"
    "github.com/google/uuid"  // go get github.com/google/uuid
)

func main() {
    id := uuid.New()
    fmt.Println(id)  // e.g. 7c9e6679-7425-40de-944b-e07fc1f90ae7
}

use Ramsey\Uuid\Uuid;  // composer require ramsey/uuid

$id = Uuid::uuid4()->toString();
echo $id;  // e.g. 7c9e6679-7425-40de-944b-e07fc1f90ae7

require 'securerandom'

id = SecureRandom.uuid
puts id  # e.g. 7c9e6679-7425-40de-944b-e07fc1f90ae7

# Single UUID
curl uuid4.com

# Multiple UUIDs
curl "uuid4.com/?count=5"

# JSON format
curl "uuid4.com/?format=json"

Frequently Asked Questions

Is a UUID v4 truly unique?

For all practical purposes, yes. With 122 random bits there are over 5.3 × 10³⁶ possible values. You would need to generate about 2.7 × 10¹⁸ UUIDs (2.7 quintillion) before there is even a 50% chance of a single collision. To put that in perspective, if you generated one billion UUIDs per second it would take over 85 years to reach that point.

Can I use UUIDs as database primary keys?

Yes, and it's a common pattern. The main trade-off is that UUID v4 values are random, so they don't sort chronologically and can cause B-tree index fragmentation in some databases. If sort order matters, consider UUID v7 which embeds a timestamp. Many databases (PostgreSQL, MySQL 8+) have native UUID column types that store them efficiently as 16 bytes rather than 36-character strings.

What is the collision probability?

Vanishingly small. The probability of generating two identical UUID v4 values is approximately 1 in 2¹²². In concrete terms: if every person on Earth generated 600 million UUIDs, the probability of any two being the same would be about 50%. You are significantly more likely to be struck by a meteorite than to generate a duplicate UUID.

UUID vs ULID vs Nano ID?

UUID v4 is the universal standard - supported everywhere, well understood, and defined by RFC 4122. ULID (Universally Unique Lexicographically Sortable Identifier) adds time-ordering and is Crockford Base32 encoded, making it more compact and sortable. Nano ID is a smaller, URL-friendly alternative often used in frontend applications. For most backend and API use cases, UUID v4 remains the safest choice due to its ubiquity.

Are UUID v4 values sequential or predictable?

No. UUID v4 is generated from a cryptographically secure random number generator (CSPRNG). There is no pattern, sequence, or way to predict the next UUID from previous ones. This makes them suitable for use as security tokens, though for high-security applications a purpose-built token generator may be preferable.

How are UUID v4 values generated?

A UUID v4 is produced by generating 128 random bits from a cryptographically secure source, then setting 4 bits to the version number (0100 for v4) and 2 bits to the variant (10 for RFC 4122). The remaining 122 bits are pure randomness. The result is formatted as 32 hex digits in the 8-4-4-4-12 pattern.

What RFC defines UUIDs?

RFC 4122 (2005) originally defined UUID versions 1-5. RFC 9562 (2024) supersedes it, adding versions 6, 7, and 8 while maintaining backwards compatibility with existing UUID versions.

About This Site

Built during a hack day as a way to provide "UUIDs as a service" - a quick way to demonstrate and generate UUIDs for use in code samples without needing a library. This service is not intended for serious or critical use.

Built by Alun Davey. Ads are included with the hope of covering domain and hosting costs.

API Usage