Question 1

Could two UUIDs ever be the same?

Accepted Answer

In theory yes, in practice no. At a million generated identifiers the chance of a duplicate is 9.4 × 10⁻²⁶. The real risk is not arithmetic but the randomness source: a server that boots without enough entropy can repeat a value regardless of the formula.

Question 2

Why is the 13th digit always a 4?

Accepted Answer

It is the version number. Version 4 forces the top four bits of the seventh byte to 0100, which in hexadecimal reads as a 4. Two variant bits are pinned the same way at the 17th position, which is why only 8, 9, a and b turn up there. Counting the hyphens, those are the 15th and 20th characters.

Question 3

Should I use a UUID as a database primary key?

Accepted Answer

As a key, yes; as a sort order, no. Version 4 is entirely random, so each new row lands somewhere different in the index, and on a large table that costs write throughput. Version 7, which puts a timestamp in front, was designed for exactly this case.

Question 4

What is the difference between UUID and GUID?

Accepted Answer

Nothing substantive: GUID is the Microsoft name for the same thing. The conventions differ — Microsoft documentation often prints them uppercase and sometimes wraps them in braces.

Question 5

Are these safe to use as security tokens?

Accepted Answer

As an unguessable identifier for a shared link, yes. As an access key or a session marker, no: that randomness belongs on the server, with its own rules for issuing and revoking, rather than arriving from a browser.

Question 6

Can I regenerate the same UUID later?

Accepted Answer

Not on purpose: every run draws fresh bytes from the browser's cryptographic source, and earlier ones cannot be recalled. A chance repeat is not impossible — the table above shows how unlikely. If you need a reproducible identifier — two services deriving the same key from the same data, say — use a version 5 UUID, computed from a namespace and a name so that the same inputs always give the same UUID. This generator produces random version 4 UUIDs only.

Group	Digits	What it holds
1	8	random
2	4	random
3	4	starts with 4 — the version
4	4	starts with 8, 9, a or b — the variant
5	12	random

UUIDs generated	Chance that two match
1 million	9.4 × 10⁻²⁶
1 billion	9.4 × 10⁻²⁰
1 trillion (10¹²)	9.4 × 10⁻¹⁴
103 trillion	one in a billion
2.7 quintillion (10¹⁸)	50 per cent

UUID Generator

Result

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