ARFF File

What are ARFF files and how to open them

Are you having problems opening an ARFF file or are you simply curious about its contents? We're here to explain the properties of these files and provide you with software that can open or handle your ARFF files.

What is an ARFF file?

An .ARFF file is an Attribute-Relation File Format file.

Attribute-Relation File Format files are plain text datasets used to represent collections of similar records (instances) for tasks such as training or testing machine learning models with Weka. They describe what each record is about, the kinds of fields it has (attributes), and then provide the actual data values. The format supports attribute types like numeric, nominal (from a fixed list), string, date, or relational (for more complex, multi-instance data) so that algorithms know the kinds of data they're working with.

Such a file is written by Weka's tools or by any program aiming to supply data into Weka. It is frequently used as input for data preprocessing, classification, clustering or other analysis in Weka. The file begins with a header section defining the relation name and its attributes (with their data types), followed by a data section listing instances. Missing values can be marked with a question mark. Nominal values are listed inside braces, string values require quotes if they contain spaces or special characters, date values use a format string. Sparse datasets may omit zero values and instead list only non-zero attributes with their indices for efficiency.

Internally the format is plain ASCII and includes specific keywords that identify the sections and fields. A file will include the terms RELATION, ATTRIBUTE and DATA (preceded by @), often begin with an @RELATION declaration, followed by several @ATTRIBUTE declarations, then @DATA before the list of instances.

Byte-signature features include an ASCII 0x40 byte ("@") at the very start of the file, plus occurrence of the uppercase keywords "ATTRIBUTE", "RELATION" and "DATA" somewhere in the file. (These help tools recognise that the file is in this format.)

How to open ARFF files

Important: Different programs may use files with the ARFF file extension for different purposes, so unless you are sure which format your ARFF file is, you may need to try a few different programs.

While we have not verified the app ourselves yet, our users have suggested a single ARFF opener which you will find listed below.

Last updated: August 20, 2026

Various apps that use files with this extension

These apps are known to open certain types of ARFF files. Remember, different programs may use ARFF files for different purposes, so you may need to try out a few of them to be able to open your specific file.

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