PlantUML Text Encoding


PlantUML defines a standardized way to encode diagram text description to a simple string of characters that contains only digits, letters, underscore and minus character. The goal of such an encoding is to facilitate communication of diagrams through URL (see server). This encoding includes compression to keep encoded strings as short as possible.

The encoded metadata is stored in the generated PNG, so the diagram source can be extracted from the diagram itself! (see server#metadata).


Deflate algorithm is used by default.

You can also use simple HEX encoding, see below. An initial ~h is added to indicate this encoding.


For example, the following uml text description:

Alice -> Bob: Authentication Request
Bob --> Alice: Authentication Response

is encoded as:


To achieve such encoding, the text diagram is:

  1. Encoded in UTF-8
  2. Compressed using Deflate algorithm
  3. Reencoded in ASCII using a transformation close to base64

Why not use Base64?

The main reason is historic: this format was not created to be public at first. Now, it's too late to change it. However, the only difference is in character order.

Where in base64 the mapping array for values 0-63 is:


For PlantUML, the mapping array for values 0-63 is:


Compression comparison

The following diagram:

🎉 Copied!

skinparam backgroundColor #EEEBDC
skinparam handwritten true

skinparam sequenceArrowColor DeepSkyBlue
skinparam sequenceActorBorderColor DeepSkyBlue
skinparam sequenceLifeLineBorderColor blue
skinparam sequenceLifeLineBackgroundColor #A9DCDF
skinparam sequenceParticipantBorderColor DeepSkyBlue
skinparam sequenceParticipantBackgroundColor DodgerBlue
skinparam sequenceParticipantFontName Impact
skinparam sequenceParticipantFontSize 17
skinparam sequenceParticipantFontColor #A9DCDF
skinparam sequenceActorBackgroundColor aqua
skinparam sequenceActorFontColor DeepSkyBlue
skinparam sequenceActorFontSize 17
skinparam sequenceActorFontName Aapex

actor User
participant "First Class" as ParticipantA
participant "Second Class" as ParticipantB
participant "Last Class" as ParticipantC

User -> ParticipantA: DoWork
activate ParticipantA

ParticipantA -> ParticipantB: Create Request
activate ParticipantB

ParticipantB -> ParticipantC: DoWork
activate ParticipantC
ParticipantC --> ParticipantB: WorkDone
destroy ParticipantC

ParticipantB --> ParticipantA: Request Created
deactivate ParticipantB

ParticipantA --> User: Done
deactivate ParticipantA

is compressed to 428-char string length using Deflate


You can use -encodeurl or -decodeurl in the command line flags to encode or decode the text.

Implementations of the encoder in various languages:

Simple HEX format

If you find Deflate and Brotli too complex, you can try the HEX format. In that case, you just have to encode each character in hexadecimal format.

For example :

Alice->Bob : I am using hex

will be turned into:


To indicate the use of HEX format, you must add ~h at the start of the data sent to PlantUML server.

Since there is no compression here, the URL will become very long as the diagram grows.

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