We are happy to present a short description of ARFF format that is very useful for those interested in machine learning. In this post we shall explain some features of this format.

What is ARFF?

ARFF stands for Attribute-Relation File Format. It is an ASCII text file that describes a list of instances sharing a set of attributes. ARFF files were developed by the Machine Learning Project at the Department of Computer Science of The University of Waikato for use with the Weka machine learning software. This document descibes the version of ARFF used with Weka versions 3.2 to 3.3; this is an extension of the ARFF format as described in the data mining book written by Ian H. Witten and Eibe Frank (the new additions are string attributes, date attributes, and sparse instances).

This explanation was cobbled together by Gordon Paynter (gordon.paynter at ucr.edu) from the Weka 2.1 ARFF description, email from Len Trigg (lenbok at myrealbox.com) and Eibe Frank (eibe at cs.waikato.ac.nz), and some datasets. It has been edited by Richard Kirkby (rkirkby at cs.waikato.ac.nz). Contact Len if you're interested in seeing the ARFF 3 proposal.


ARFF files have two distinct sections. The first section is the Header information, which is followed by the Data information.

The Header of the ARFF file contains the name of the relation, a list of the attributes (the columns in the data), and their types. An example header on the standard IRIS dataset looks like this:

% 1. Title: Iris Plants Database % % 2. Sources: % (a) Creator: R.A. Fisher % (b) Donor: Michael Marshall (MARSHALL%PLU@io.arc.nasa.gov) % (c) Date: July, 1988 % @RELATION iris

@ATTRIBUTE sepallength NUMERIC @ATTRIBUTE sepalwidth NUMERIC @ATTRIBUTE petallength NUMERIC @ATTRIBUTE petalwidth NUMERIC @ATTRIBUTE class Iris-setosa,Iris-versicolor,Iris-virginica

The Data of the ARFF file looks like the following:

@DATA 5.1,3.5,1.4,0.2,Iris-setosa 4.9,3.0,1.4,0.2,Iris-setosa 4.7,3.2,1.3,0.2,Iris-setosa 4.6,3.1,1.5,0.2,Iris-setosa 5.0,3.6,1.4,0.2,Iris-setosa 5.4,3.9,1.7,0.4,Iris-setosa 4.6,3.4,1.4,0.3,Iris-setosa 5.0,3.4,1.5,0.2,Iris-setosa 4.4,2.9,1.4,0.2,Iris-setosa 4.9,3.1,1.5,0.1,Iris-setosa

Lines that begin with a % are comments. The @RELATION, @ATTRIBUTE and @DATA declarations are case insensitive.

Datahub examples

You can find ARFF files in resources for our machine-learning datasets datahub.io/machine-learning that we extracted from openml website. We hope that you will find them useful for your projects in macahine learning and data science.

Branko graduated physics at the University of Belgrade and his current work focuses on finding, extracting, formatting and publishing data on DataHub. In his free time he enjoys reading, coding and long walks on the beach in his landlocked home country of Serbia