How Long Is a UUID?

What is a How Long Is a UUID?

A UUID is a 128-bit value. Written the canonical way it becomes 36 characters: five groups of 8, 4, 4, 4 and 12 hexadecimal digits separated by four hyphens. Remove the hyphens and you have 32 characters, which is exactly 32 × 4 bits = 128 bits with nothing lost or added.

Stored natively it is 16 bytes — PostgreSQL's uuid, SQL Server's uniqueidentifier, MySQL's BINARY(16). That is the form to prefer: it is less than half the size of the text form and compares much faster, because the database can compare 16 bytes instead of parsing a 36-character string.

Not all 128 bits are payload. Six of them are structure: four bits record the version and two record the variant, leaving 122 random bits in a v4 UUID. That is still an enormous space — 5.3 × 10³⁶ possible values — but it explains why the entropy of a v4 is quoted as 122 and not 128.

How to

1. Copy the canonical 36-character form for humans, docs and configuration files.
2. Drop the hyphens for logs, compact keys and anything that has to fit a fixed width.
3. Store 16 bytes natively when the column is indexed, and read the length breakdown below when someone asks why a UUID is not 128 characters long.

Use cases

• Column sizing: CHAR(36) versus a native 16-byte type — the difference decides your index size.
• Fixed-width logs, where you need to know whether a field is 32 or 36 characters before you write a parser.
• UI decisions: knowing a v4 is 36 characters tells you whether to truncate it in a table cell or show it in full.

Compare How Long Is a UUID? with other formats

Code examples

Measuring the length in code

import uuid

u = uuid.uuid4()
len(str(u))      # 36
len(u.hex)       # 32
len(u.bytes)     # 16, bytes
u.int            # the 128-bit integer value

# Base64url is the shortest lossless text form
import base64
len(base64.urlsafe_b64encode(u.bytes).rstrip(b'='))   # 22

Sizing a column

-- PostgreSQL: native type, 16 bytes on disk
CREATE TABLE items (id uuid PRIMARY KEY DEFAULT gen_random_uuid());

-- MySQL: BINARY(16) instead of CHAR(36)
CREATE TABLE items (id BINARY(16) PRIMARY KEY);

-- SQL Server: UNIQUEIDENTIFIER instead of VARCHAR(36)
CREATE TABLE items (id UNIQUEIDENTIFIER NOT NULL DEFAULT NEWID());

Shortening a UUID

// 36 characters -> 22, with no loss at all
const hex = '550e8400e29b41d4a716446655440000';
const b64 = Buffer.from(hex, 'hex').toString('base64url');
console.log(b64.length);   // 22

Frequently asked questions