What Is a UUID? How to Generate UUID v4 (and When to Use It)

A UUID (Universally Unique Identifier) is a 128-bit number used to label information without a central authority handing out IDs. It's why two different systems can each generate IDs independently and still be confident they'll never collide. You've almost certainly seen one — they look like this:

550e8400-e29b-41d4-a716-446655440000

What UUID stands for

The name says it all: a UUID is meant to be unique across all time and space, without any coordination. That's different from a database auto-increment column (like 1, 2, 3), which is unique only within one table and reveals information about how many records exist.

Microsoft calls the same concept a GUID (Globally Unique Identifier). For practical purposes, UUID and GUID are interchangeable — they refer to the same kind of value.

UUID versions: v1, v4 and friends

Not all UUIDs are created the same way. The number after the "v" tells you how the ID was generated:

UUID v1 — based on the current timestamp and the machine's MAC address. Sortable over time, but it leaks when and where the ID was made.
UUID v4 — generated from random numbers. No timestamp, no machine info, and the most common choice for general use.
UUID v5 — generated from a name (deterministic). The same name always produces the same UUID.

When someone says "generate a UUID" without specifying, they almost always mean v4.

Can two UUIDs collide?

The short answer: practically, no. A v4 UUID has 122 bits of randomness. To have a 50% chance of finding a single collision, you'd need to generate about 2.7 × 10¹⁸ of them — roughly a billion UUIDs every second for 85 years. The odds are so low that collision is treated as impossible in real-world systems.

When to use a UUID

Database primary keys — especially in distributed systems or when IDs are generated before saving.
Session IDs and API tokens — an unguessable ID for a user session or record.
Public identifiers — exposing /users/550e8400-… instead of /users/42 hides how many records you have.

How to generate a UUID v4

You can generate one instantly with our free UUID Generator:

1.Open the generator and choose how many UUIDs you need.
2.Click Generate to create cryptographically random v4 UUIDs.
3.Copy the result straight into your code, database, or configuration.

The generation happens locally in your browser using a secure random source — nothing is sent to a server, so the IDs are truly private.

Frequently asked questions

Is a UUID truly unique?

In practice, yes. v4 UUIDs use enough randomness that a collision is effectively impossible. They aren't guaranteed unique the way a central counter is, but the probability of a duplicate is so small it's treated as zero.

What's the difference between UUID and GUID?

They're the same thing. GUID is Microsoft's name for the same 128-bit identifier format; UUID is the more general, standards-based term (RFC 4122).

Should I use UUID v1 or v4?

Use v4 for most cases — it's simpler and doesn't reveal timestamps or hardware addresses. Choose v1 only if you need IDs that sort roughly by creation time.

Need a UUID right now?

Generate as many cryptographically secure UUID v4s as you need with the free UUID Generator.

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