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+).


In an attempt to make this software self-sustaining and actively developed in the future, new binaries are available to subscribers only:

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 for about 10 distros (incl. Ubuntu, Fedora, Suse), 32- and 64-bit.

Subscribers have access to all versions of the program and to e-mail support. The program can be installed on many computers and can be also used after the end of subscription.

Fityk is usually bought by companies and institutions. If you’re a home user or a student in an institution that won’t buy software for you - email wojdyr@gmail.com for info about discount.

Versions 0.9.8 and older are free: for Windows and Linux (Ubuntu PPA and RPMs from OBS).

Source code: GitHub (releases)

Version 1.3.1 was released on 2016-12-19 (changelog). New version notifications are delivered via email and feeds.


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.