UUID Versions Explained: v1 vs v4 vs v7 — Which Should You Use?

What is a UUID?

A UUID (Universally Unique Identifier) is a 128-bit value designed to be unique across space and time without requiring a central coordinating authority. Standardized in RFC 9562 (which replaced the older RFC 4122 in 2024), UUIDs are displayed as 32 hexadecimal characters in a specific 8-4-4-4-12 format:

550e8400-e29b-41d4-a716-446655440000
^^^^^^^^ ^^^^ ^^^^ ^^^^ ^^^^^^^^^^^^
time-low  mid  hi   var    node
              +ver

The version number is encoded in the first nibble of the third group (bits 48-51). The variant field in the fourth group indicates which UUID specification the value conforms to. For the standard variant (RFC 9562), the first one or two bits of this field are 10 in binary, meaning it always starts with 8, 9, a, or b.

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UUID Versions at a Glance

There are eight UUID versions defined in RFC 9562. In practice, three are commonly used: v1, v4, and the newer v7. Here is an overview of all versions:

UUID v1: Timestamp + MAC Address

UUID v1 combines a 60-bit timestamp (100-nanosecond intervals since October 15, 1582) with the 48-bit MAC address of the generating machine and a 14-bit clock sequence to handle rapid generation.

Example UUID v1: 6ba7b810-9dad-11d1-80b4-00c04fd430c8
                        ^                ^^^^^^^^^^^^
                      version=1          MAC address

Problems with UUID v1

• Privacy risk: The MAC address reveals the generating machine's hardware identity
• Not naturally sortable: The timestamp bits are not in the most-significant position, so lexicographic sorting does not match chronological order
• Clock dependency: Requires a stable system clock; clock rollbacks can cause collisions

UUID v1 is still found in legacy systems (Cassandra, for example, has historically used time-based UUIDs) but should be avoided in new applications due to privacy concerns.

UUID v4: Random

UUID v4 is the most popular version. It fills 122 of its 128 bits with cryptographically random data (6 bits are reserved for version and variant). This makes v4 UUIDs simple, private, and unpredictable.

// Generate UUID v4 in JavaScript (modern)
const id = crypto.randomUUID();
// => "f47ac10b-58cc-4372-a567-0e02b2c3d479"
//                  ^ version = 4

// Generate UUID v4 in Python
import uuid
id = uuid.uuid4()
// => UUID('a3bb189e-8bf9-3888-9912-ace4e6543002')

Strengths of UUID v4

• Privacy-safe: No embedded hardware or timestamp information
• Simple to generate: Only requires a good random number generator
• Widely supported: Available in every language and platform
• Unpredictable: Cannot guess adjacent IDs from a known ID

Weaknesses of UUID v4

• Not sortable: Random distribution causes B-tree index fragmentation in databases
• No timestamp: You cannot extract creation time from the ID
• Large: 16 bytes vs 8 bytes for a 64-bit integer

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UUID v7: The Best of Both Worlds

UUID v7, introduced in RFC 9562 (2024), combines a 48-bit Unix timestamp in milliseconds with 74 bits of randomness. The timestamp occupies the most significant bits, making v7 UUIDs naturally sortable in chronological order.

UUID v7 structure (128 bits):
┌──────────────────────┬────┬──────────┬──┬───────────────────────┐
│   unix_ts_ms (48)    │ver │ rand_a   │va│      rand_b (62)      │
│                      │(4) │  (12)    │(2)│                       │
└──────────────────────┴────┴──────────┴──┴───────────────────────┘

Example: 018f6a7c-d1e0-7b3a-9c4d-8e2f1a3b5c7d
         ^^^^^^^^^^^      ^
         timestamp        version = 7

Why UUID v7 is Recommended for New Projects

UUID v5: Deterministic, Name-Based

UUID v5 generates a consistent UUID from a namespace UUID and a name string by hashing them with SHA-1. The same inputs always produce the same UUID, making v5 ideal for generating stable identifiers from known data.

// UUID v5 from URL namespace
import { v5 as uuidv5 } from 'uuid';

const DNS_NAMESPACE = '6ba7b810-9dad-11d1-80b4-00c04fd430c8';
const id = uuidv5('example.com', DNS_NAMESPACE);
// Always produces: cfbff0d1-9375-5685-968c-48ce8b15ae17

Use v5 when you need to generate the same UUID from the same input data consistently, such as creating deterministic IDs for content-addressable storage, URL-to-ID mapping, or deduplication systems.

Security Considerations

UUIDs are identifiers, not security tokens. Here is what to keep in mind:

Which UUID Version Should You Use?

Use UUID v5 when you need deterministic IDs derived from known inputs. Avoid UUID v1 in new applications. Use our UUID generator to create UUIDs instantly for testing and development.

Storing UUIDs in Databases

How you store UUIDs significantly impacts query performance and storage size:

-- PostgreSQL: Native UUID type (16 bytes, optimized)
CREATE TABLE users (
  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
  name TEXT NOT NULL
);

-- MySQL 8.0+: BINARY(16) for performance
CREATE TABLE users (
  id BINARY(16) PRIMARY KEY,
  name VARCHAR(255) NOT NULL
);
-- Use UUID_TO_BIN() and BIN_TO_UUID() for conversion

-- Avoid: VARCHAR(36) — wastes 36 bytes vs 16 bytes
-- and prevents binary comparison optimizations

Always store UUIDs in native UUID or BINARY(16) columns. Never use VARCHAR(36) in production because it doubles storage requirements and slows index lookups due to character-by-character comparison instead of binary comparison.

Generate UUIDs for Your Project

Understanding UUID versions helps you make informed decisions about identifiers in your applications. For new projects, UUID v7 offers the best combination of uniqueness, sortability, and database performance. For existing systems, UUID v4 remains an excellent choice that is supported everywhere.

Bookmark our UUID generator for quick access whenever you need identifiers for testing or development. Explore our other developer tools including a JSON formatter and Base64 encoder.

Frequently Asked Questions