What a UUID is

A UUID (Universally Unique Identifier, also called a GUID) is a 128-bit value written as 32 hexadecimal digits in five groups: 8-4-4-4-12. RFC 9562 (2024, replacing RFC 4122) defines eight versions. Two of them matter for new systems: v4, which is random, and v7, which starts with a timestamp so that values sort by creation time.

Uniqueness is probabilistic. A v4 UUID has 122 random bits; you would need to generate about 2⁶¹ of them (2.3 quintillion) before the chance of a single collision reaches 50%. No coordination between generators is required.

v4 or v7

	v4	v7
Content	122 random bits	48-bit Unix millisecond timestamp, then 74 bits of counter and random data
Sortable by time	No	Yes, lexically and as bytes
Reveals creation time	No	Yes, to the millisecond
Database index behaviour	Random inserts scatter across a B-tree, which fragments the index and hurts write throughput at scale	Mostly append-only inserts, similar to an auto-increment key
Choose when	The ID must not leak timing; or the system already standardised on v4	IDs are primary keys, or you want natural time ordering in logs and storage

The v7 values generated here are monotonic within a session: when several are created in the same millisecond, a 12-bit counter in the rand_a field increments (RFC 9562 §6.2, method 1), so they still sort in generation order.

Layout

v4:  xxxxxxxx-xxxx-4xxx-Nxxx-xxxxxxxxxxxx
v7:  tttttttt-tttt-7ccc-Nrrr-rrrrrrrrrrrr

t = 48-bit Unix time in milliseconds
4 / 7 = version nibble
N = variant: 8, 9, a or b (binary 10xx)
c = 12-bit counter or random (rand_a)
x / r = random

Two special values are defined: the Nil UUID (all zeros) and the Max UUID (all f). Paste either into the inspector above.

Generating UUIDs in code

JavaScript

crypto.randomUUID();                 // v4, browsers and Node 19+
// v7: Node 24+ ships it in node:crypto; otherwise use the "uuid" package
import { v7 as uuidv7 } from "uuid";
uuidv7();

Python

import uuid
uuid.uuid4()
uuid.uuid7()        # Python 3.14+; earlier versions: pip install uuid7

SQL

-- PostgreSQL 13+
SELECT gen_random_uuid();          -- v4
SELECT uuidv7();                   -- PostgreSQL 18+
-- MySQL 8
SELECT UUID();                     -- v1 (time-based, MAC-derived node)

Command line

uuidgen                 # macOS and util-linux: v4 by default
cat /proc/sys/kernel/random/uuid

Storage

Store UUIDs as 16 raw bytes (uuid in PostgreSQL, BINARY(16) in MySQL) rather than as a 36-character string. It halves the index size and makes comparisons cheaper. Serialise to the hyphenated lowercase form at the API boundary; RFC 9562 specifies lowercase output and case-insensitive input.

Notes on this implementation

All bits come from crypto.getRandomValues, the browser's cryptographically secure generator. Nothing is derived from Math.random. Generation happens in your browser; the values are never sent anywhere and are not stored.

UUID Generator

Inspect a UUID