Excellent GUI and command-line curve fitting tool
- John Allspaw in The art of capacity planning


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Fityk [fi:tik] is a program for data processing and nonlinear curve fitting.

Primarily used

  • by scientists who analyse data from powder diffraction, chromatography, photoluminescence and photoelectron spectroscopy, infrared and Raman spectroscopy, and other experimental techniques,

  • to fit peaks – bell-shaped functions (Gaussian, Lorentzian, Voigt, Pearson VII, bifurcated Gaussian, EMG, Doniach-Sunjic, etc.),

but it is suitable for fitting any curve to 2D (x,y) data.

Features | Download | Documentation | Support


  • intuitive graphical interface (and also command line interface),

  • support for many data file formats, thanks to the xylib library,

  • dozens of built-in functions and support for user-defined functions,

  • equality constraints,

  • fitting systematic errors of the x coordinate of points (for example instrumental zero error or sample displacement correction in powder diffraction),

  • manual, graphical placement of peaks and auto-placement using peak detection algorithm,

  • various optimization methods (standard Marquardt least-squares algorithm, Genetic Algorithms, Nelder-Mead simplex),

  • handling series of datasets,

  • automation with macros (scripts) and embedded Lua for more complex scripting

  • the accuracy of nonlinear regression verified with reference datasets from NIST,

  • an add-on for powder diffraction data (Pawley refinement)

  • modular architecture,

  • open source licence (GPLv2+).


ico-win MS Windows: fityk-1.3.1-setup.exe

ico-osx Mac OS X (10.6 or later): fityk-1.3.1-osx.zip

ico-tux Linux: binary RPM and deb packages (files) for about 10 distros (incl. Ubuntu, Fedora, Suse), 32- and 64-bit.

Source code: GitHub (releases)

Installers for ver. 1.0+ used to be available to paid subscribers only. This made version 0.9.8 more popular than more recent releases. To change this situation the latest binaries are no longer paywalled. The author is grateful to all people who supported Fityk with subscriptions.

Version 1.3.1 was released on 2016-12-19 (changelog).


Citing Fityk in academic papers: M. Wojdyr, J. Appl. Cryst. 43, 1126-1128 (2010) [reprint]


Feel free to send questions, comments, requests, bug reports, and success stories. Asking for a new feature usually results in adding the request to the TODO list or, if it already is in the list, in assigning higher priority to it.