What a UUID is
A UUID (universally unique identifier) is a 128-bit value written as 32 hexadecimal digits in five hyphenated groups, such as 3f2b6a1e-8c4d-4e2a-9b7f-1a2b3c4d5e6f. Its job is to let anyone, anywhere, mint an identifier with no coordination and be confident nobody else will ever mint the same one. That is why UUIDs turn up as database primary keys, order references, session identifiers, file names and message ids across almost every modern system. Everything on this page is generated in your browser from its own cryptographic random source; nothing is created on, or sent to, a server.
How version 4 works
There are several UUID versions, and version 4 is the purely random one. Of the 128 bits, 4 are fixed to record the version (the 4 you can see in every UUID above) and 2 more mark the variant (which is why the 17th digit is always 8, 9, a or b). The remaining 122 bits are filled with cryptographically secure randomness. No clocks, no network card addresses, no counters: uniqueness rests entirely on the size of the number space.
Could two ever collide?
In theory, yes; in practice, the odds are absurd. The birthday problem says a duplicate becomes more likely than not only once you have generated about 2.7 quintillion UUIDs. To put that somewhere imaginable: a system generating one billion UUIDs every second would take about 86 years to get there, and even then it would only have reached a coin-flip chance of a single collision across the entire 86-year haul.
This is why real systems treat v4 UUIDs as unique without checking. The practical risks are not mathematical but human: a broken random source, or code that accidentally reuses an identifier. This tool avoids the first by using crypto.randomUUID where available and crypto.getRandomValues otherwise, both backed by the operating system's secure random generator, and never Math.random.
Using them well
A few practical notes. UUIDs are conventionally written in lowercase, and this tool follows that; treat comparisons as case-insensitive to be safe. They are identifiers, not secrets: they are hard to guess, but if something must be unguessable and confidential, use a proper token or password (see the password generator). Random keys also spread inserts across a database index rather than appending in order, which is usually fine but worth knowing at very high write volumes; newer schemes like UUIDv7 embed a timestamp to fix that. And if you need many at once for seeding test data or spreadsheet rows, generate up to 100 above and Copy all gives you one per line, ready to paste straight into a spreadsheet column, a SQL insert script or a JSON fixture file without any reformatting.
Frequently asked questions
What is a version 4 UUID?
A universally unique identifier built almost entirely from random bits. Of its 128 bits, 6 are fixed to mark the version and variant, leaving 122 random bits. It is the most common UUID version because it needs no central authority, no clock and no hardware address, just a good source of randomness.
Are these UUIDs really unique?
Not guaranteed, but collisions are so unlikely they are ignored in practice. There are about 5.3 undecillion possible v4 UUIDs, and you would need to generate roughly 2.7 quintillion of them before a duplicate became more likely than not. Every database and cloud platform in production relies on those odds.
How random are they?
The tool uses crypto.randomUUID where the browser provides it, falling back to crypto.getRandomValues, both of which draw from the operating system's cryptographically secure random source. It never uses Math.random, which is predictable and unsuitable for identifiers.
Why do all the UUIDs have a 4 in the same place?
The 13th hexadecimal digit is the version field, so every version 4 UUID shows a 4 there. The 17th digit is the variant field and is always 8, 9, a or b. Those are the only positions that are not random.
Are the UUIDs generated on a server?
No. They are generated in your browser using its own random source, and nothing is sent or stored anywhere. Refreshing the page produces a completely fresh, unrecorded set.