UUID generator: v4, v7, ULID and GUID

Every UUID is 128 bits. The version says how those bits were made. RFC 9562 (May 2024) replaced RFC 4122, kept versions 1 to 5 and added 6, 7 and 8. ULID and Nano ID are not UUIDs, but they solve the same problem and are included here because people often compare them.

A v4 UUID is fully random, so each new row lands at a random place in a B-tree index. On large tables that means more page splits and a cache that holds less of the index. A v7 UUID starts with the current time in milliseconds, so new rows are added near the end of the index, much like an auto-increment number.

Within one millisecond RFC 9562 lets a generator add a counter or increase the random part so the order still holds. This tool does that: a batch of 10,000 v7 UUIDs made in the same millisecond is still in sorted order. The cost of v7 is that the creation time is readable by anyone who sees the ID.

Many tools now make v7 natively: uuidv7() in PostgreSQL 18, uuid.uuid7() in Python 3.14, Guid.CreateVersion7() in .NET 9 and uuid.NewV7() in Go's google/uuid.

A UUID is written as 32 hex digits in five groups, 8-4-4-4-12. Two digits tell you what kind it is without decoding anything:

018f3a9c-7d2e-7b41-9f0c-3e5a8d1b2c47
              ^    ^
              |    variant: 8, 9, a or b = the RFC 9562 layout
              version: 7

The first digit of the third group is the version (1 to 8). The first digit of the fourth group holds the variant; for standard UUIDs it is always 8, 9, a or b. A string with a c or d there is an old Microsoft-variant GUID, and a 0 to 7 is the even older NCS variant. The decoder above checks both digits and pulls out the timestamp for v1, v6, v7 and ULID.

Most languages have a built-in way to make a v4 UUID, and newer versions add v7.

// JavaScript (browsers in secure contexts, Node.js 19+ as a global)
crypto.randomUUID();                 // v4

# Python
import uuid
uuid.uuid4()                         # random
uuid.uuid5(uuid.NAMESPACE_DNS, "example.com")
uuid.uuid7()                         # Python 3.14+

// Go (github.com/google/uuid)
id := uuid.New()                     // v4
id7, err := uuid.NewV7()

-- PostgreSQL
SELECT gen_random_uuid();            -- v4, built in since 13
SELECT uuidv7();                     -- PostgreSQL 18+

// C#
Guid.NewGuid();                      // v4
Guid.CreateVersion7();               // .NET 9+

# Shell (macOS prints uppercase, Linux lowercase)
uuidgen
cat /proc/sys/kernel/random/uuid     # Linux

For ULIDs and Nano IDs, use the ulid and nanoid packages in JavaScript. If you need UUIDs as test data in bulk, the mock data generator can put them in a column next to names, emails and dates.

A GUID is the same 128-bit value as a UUID, and the text form is identical apart from the habit of braces and uppercase, which the {GUID} format and the Uppercase switch above give you. The trap is binary: .NET's Guid.ToByteArray() and SQL Server write the first three groups in little-endian order.

Text:          00112233-4455-6677-8899-aabbccddeeff
RFC 9562 bytes: 00 11 22 33 44 55 66 77 88 99 aa bb cc dd ee ff
.NET bytes:     33 22 11 00 55 44 77 66 88 99 aa bb cc dd ee ff

So if a UUID looks scrambled after moving between a .NET service and another system through a 16-byte binary column, the first 8 bytes were swapped. Pass UUIDs as text between systems, or convert the byte order on purpose. SQL Server also sorts uniqueidentifier values by the last group first, so v7 values do not sort by time there unless you store them another way.

A v4 UUID has 122 random bits, which is about 5.3 × 10³⁶ possible values. By the birthday rule, the number of IDs you need before there is a 50% chance of any duplicate is about √(2 · ln 2 · 2¹²²), roughly 2.7 × 10¹⁸. Generating a billion per second, that takes about 85 years.

v7 has fewer random bits (74) but they only need to be unique within the same millisecond, which is a far smaller pool. A Nano ID with the default 21 characters from 64 symbols has 126 random bits, slightly more than v4. Shorter Nano IDs lose 6 bits per character removed; the tool shows the bit count as you change the length. For generating secrets rather than IDs, use the password generator; to turn a v7 or ULID time into a date, the Unix timestamp converter reads milliseconds too. Building a data-heavy app? Browse database tools on DevHunt.