UUID vs GUID — What's the Difference?

Published by UUID Generator Team

If you've worked with unique identifiers in software, you've likely seen both UUID and GUID. They look the same: a 36-character string like 550e8400-e29b-41d4-a716-446655440000. So what's the difference?

Definitions

UUID stands for Universally Unique Identifier. It's defined by the IETF in RFC 4122 and has a standard 128-bit format with five groups of hex digits.

GUID stands for Globally Unique Identifier. It's Microsoft's term for the same concept. In practice, GUIDs are UUIDs. Microsoft adopted the UUID format for COM, Windows Registry, and .NET.

Technical Overlap

Both UUID and GUID refer to the same 128-bit value. The format is identical: 8-4-4-4-12 hex characters. When you generate a GUID in C# with Guid.NewGuid(), you get a Version 4 (random) UUID. When you generate a UUID with our generator or API, you get a value that is also a valid GUID.

Byte Order (Variants)

Historically, some Microsoft systems used a different byte order for the first three groups (time-low, time-mid, time-high). That variant is mostly legacy. Today, GUIDs produced by Windows and .NET are RFC 4122 compliant and match UUIDs. Our validator accepts both and reports the variant.

When to Use Which Term

Use UUID when you're in a cross-platform or standards-based context (APIs, databases, REST, RFCs). Use GUID when you're in a Microsoft ecosystem (C#, COM, Windows). The underlying value is the same; the name is just convention.

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