UUID v4 Generator

What a UUID v4 is

Version 4 is the random UUID. Of its 128 bits, 122 come straight from a random number generator. The other six are fixed by the standard: four bits say "this is version 4" and two bits mark the variant. That is why every v4 UUID has a 4 as the first character of the third group and an 8, 9, a or b as the first character of the fourth group.

Because nothing in a v4 UUID is derived from time, a machine or a name, it carries no information at all. Two v4 UUIDs made a second apart on the same laptop look no more related than two made on different continents.

How this generator makes them

The page uses the browser's cryptographic random source, crypto.getRandomValues, through the same uuid library that most Node projects use. The values are made on your machine and never sent anywhere. You can generate one at a time or up to 1000 in a batch, then copy them all or download them as a text, JSON or CSV file. The format options change how each value is displayed: uppercase for C# and SQL Server habits, braces for the Windows registry style, no hyphens when a system wants 32 plain hex characters, and Base64 for a shorter 22 character form.

When to use v4

• Any identifier that only needs to be unique. Request ids, session ids, upload filenames, correlation ids in logs, idempotency keys.
• Ids created on many machines with no coordination. Clients, mobile apps and edge functions can all mint ids that will not clash with the server's.
• When the id must not reveal anything. A v4 UUID does not leak when or where it was made.

Two cases where another version is the better tool. If the UUID becomes a database primary key on a table with heavy inserts, a v7 UUID keeps the index in order and the writes fast. If you need the same input to always produce the same UUID, that is what v5 is for.

How unique a v4 UUID is

There are 2 to the power 122 possible values, about 5.3 undecillion. Using the birthday paradox, you would need to generate roughly 100 trillion UUIDs before the chance of a single duplicate reaches one in a billion. For every practical system the answer is that collisions do not happen, as long as the random source is a real one. The full maths is in our post on UUID collision probability.

Anatomy of an example

f47ac10b-58cc-4372-a567-0e02b2c3d479
                 ^    ^
                 |    variant: 8, 9, a or b
                 version: always 4

Everything else in the value is random. The hyphens are only there for readability. Systems that store UUIDs as 16 raw bytes drop them.

Generate a v4 UUID in code

// JavaScript, browsers and Node 19+
const id = crypto.randomUUID();

# Python
import uuid
str(uuid.uuid4())

-- PostgreSQL 13+
SELECT gen_random_uuid();

Every major language has a one liner. We keep a page for each: JavaScript, Python, Java, Go, C#, PHP, Ruby, Rust, PostgreSQL and MySQL.

Frequently asked questions

Is a v4 UUID safe to use as a secret token?

It is random enough, 122 bits from a cryptographic source, but UUIDs are designed to be identifiers, not secrets. They get logged, put in URLs and shown in error messages. For password reset links or API keys, generate a dedicated random token and keep it out of logs.

Can I use v4 UUIDs as a database primary key?

Yes, and many teams do. The cost is that random values scatter inserts across the whole index, which slows writes on large tables. If you are choosing today, v7 gives the same uniqueness with time ordering. Our post on UUIDs as primary keys covers the trade offs.

Why does the third group always start with 4?

That character holds the version number. The standard reserves four bits at that position so any parser can tell which version it is looking at. The first character of the fourth group is the variant and is always 8, 9, a or b for RFC UUIDs.

Is UUID v4 the same as GUID?

Yes. GUID is Microsoft's name for the same 128 bit value. Guid.NewGuid() in .NET produces a v4 UUID. The only differences are habits: GUIDs are often shown in uppercase and sometimes wrapped in braces, both of which this generator can do.